MFactory Knowledge Bank: Gear Oil: The Myth

Gear Oil: The Myth

One of the most commonly brought up questions from our customers is what gear oil to use in their freshly built Honda Transmission. Our reply to this question has always been: DON'T!

"He must be crazy", you are probably thinking, but there is a simple explanation for this answer:

After a comprehensive analysis, we can conclude that 90% of drivers out there use the completely wrong fluid. The lubrication system in the Honda transmission was designed to use engine oil, and this has not been changed for any of their transmissions. What this means is that, in order for the gears, shafts, bearings, hubs/sliders to be lubricated as intended, a low viscosity transmission fluid must be used, regardless of whether it is synchronous engagement or dog engagement.

Basically, you MUST use the correct transmission fluid in a Honda Transmission and should not use a performance gear oil as it is too thick and does more damage than good. Performance Gear Oils also lack the additives necessary for the smooth running of the synchronous engagement system, as well as a lot of them containining chemicals which disintegrate the brass synchro rings. This is not common knowledge because customers are led to believe marketing and/or technical advice related to your typically large hypoid transmissions, NOT your specialist Honda Transmission.

For most cases where the vehicle is an unmodified Street Car, the OEM Honda MTF (Manual Transmission Fluid) is more than adequate. The problem arises when you start putting either higher power or higher rpm shifting through the transmission. Not only must you still keep within the tolerances of the oem lubrication system, you must also protect the transmission from shock loads. Using Performance Gear Oil is the Number 2 Reason (after incorrect installation) that is directly responsible for most transmission failures (i.e the Cause), subsequently leading to a part failure (i.e the Symptom).

Unfortunately, most consumers tend to follow the advice of your so-called "Performance Specialist" who are only concerned about the symptoms, rather than the cause, and usually end up recommending big-brand (and often expensive) performance gear oils. This is a 100% NO GO on a Honda Transmission. Although GL4 & GL5 Rated Gear Oils contain shock protection additives, they are also too thick for the oem Honda lubrication system. The engagement system (synchronous vs dog) is irrelevant in this case. Lubrication is the key word here.

A basic analogy would be: Oil is your transmissions blood supply and needs to flow smoothly. Clog it up with cholesterol (i.e thick gear oil), and it will have a heart attack. Not enough Iron (i.e shock additives), and it will get anaemic/weak.

So what do we recommend you to lubricate your transmission with? Below is a guide that we highly recommend following:

Level 1) Stock Power. Street Use
Requirements - Low Viscosity Transmission Fluid
Choices - Honda MTF, Torco MTF. Other transmission fluids are available

Level 2) Low to Moderate Power. Street and/or Race Use
Requirements - Low Viscosity Transmission Fluid, Limited Shock/Wear Protection & Stability, Meet GL4 Requirements
Choices - Torco MTF. Other transmission fluids are available

Level 3) High Power and/or High RPM. Street and/or Race Use
Requirements - Low Viscosity Transmission Fluid, Advanced Shock/Wear Protection & Stability, Meet GL5/GL6 Requirements
Choices - Torco RTF. This is currently the ONLY GL6 Rated Transmission Fluid on the market Specifically Designed for the Performance Honda Transmission


MFactory Competition Products - THE Replacement for Displacement!

Exactly what weight/viscosity should we be aiming for?

As in the article, The lubrication system in the Honda transmission was designed to use engine oil, so use the correct Trans fluid for the Honda Trans

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Originally Posted by e240

After a comprehensive analysis, we can conclude that 90% of drivers out there use the completely wrong fluid.

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Can you detail this "comprehensive analysis"?

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Originally Posted by e240

The lubrication system in the Honda transmission was designed to use engine oil, and this has not been changed for any of their transmissions.

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To the best of my knowledge, for some years Honda has specified their MTF for all their manual gearboxes (and retro specified this oil for boxes that pre-date MTF availability), and this is a gear specific oil, not an engine oil.

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Originally Posted by e240

What this means is that, in order for the gears, shafts, bearings, hubs/sliders to be lubricated as intended, a low viscosity transmission fluid must be used,

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Why? State a case, don't just make an unsupported statement...

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Originally Posted by e240

regardless of whether it is synchronous engagement or dog engagement.

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Dog teeth engagement on the forward gears is used on nearly all gearboxes (I am aware of some Bugatti boxes that use sliding / meshing gear tooth engagement, similar to the usual reverse gear engagement, there are most likely others). This is the case for gearboxes with and without synchromesh, the only significant difference being that synchromesh boxes tend to use smaller and more dog teeth that engage in closer fitting 'dog slots'.

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Originally Posted by e240

Basically, you MUST use the correct transmission fluid in a Honda Transmission and should not use a performance gear oil as it is too thick and does more damage than good.

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Please explain the mechanism by which this damage is created by virtue of the oil simply being thicker...

Why are you implying that all "high performance" gear oils are thick oils? This is far from being the case, the Motul gearbox oil in my car is quite light being comparable in viscosity to 10w30 engine oil (or maybe that's 15w40, can't recall offhand).

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Originally Posted by e240

Performance Gear Oils also lack the additives necessary for the smooth running of the synchronous engagement system, as well as a lot of them containining chemicals which disintegrate the brass synchro rings.

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An issue may possibly arise if an oil that is formulated for use in a non synchromesh racing box is used in a gearbox that has synchromesh. If using a gear oil ("high performance" or otherwise) that is formulated to be compatible with the metallurgy of synchromesh rings and the frictional requirements of the synchromesh action, then there ought to be no problem...

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Originally Posted by e240

This is not common knowledge because customers are led to believe marketing and/or technical advice related to your typically large hypoid transmissions, NOT your specialist Honda Transmission.

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"Hypoid transmissions"? The only place you will find 'hypoid' gearsets is the differential's crownwheel and pinion in rear wheel drive cars (and it would be ill-advised to use a hypoid oil in a gearbox).

In all gearbox applications (FWD, RWD, large, small), the gears will be 'helical' or 'straight' cut (with very rare and archaic exceptions, e.g. 'herringbone' cut). Even FWD differential crownwheels / pinions are not 'hypoid' (they are typically helical cut).

There is nothing inherantly "specialist" about Honda manual gearboxes, they are good quality but quite conventional boxes.

If you claim to be a gearbox specialist then you should know this basic stuff...

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Originally Posted by e240

For most cases where the vehicle is an unmodified Street Car, the OEM Honda MTF (Manual Transmission Fluid) is more than adequate.

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Yes. And personally I'd also not worry about using it with all but very high torque outputs and/or rpm.

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Originally Posted by e240

The problem arises when you start putting either higher power or higher rpm shifting through the transmission. Not only must you still keep within the tolerances of the oem lubrication system, you must also protect the transmission from shock loads. Using Performance Gear Oil is the Number 2 Reason (after incorrect installation) that is directly responsible for most transmission failures (i.e the Cause), subsequently leading to a part failure (i.e the Symptom).

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And you are suggesting that engine oil (an oil only 'accidentally' suitable to use in a gearbox application) will be superior in 'shock' protection to the specifically formulated and Honda recommended MTF??

You need to be careful with the term 'shock'. In this context my understanding is that it really means the oils ability to withstand momentary and sustained extreme pressures and shear loadings without compromising the oil film and thus suffering metal to metal contact (i.e. 'film strength'). It may have other explicit or implied meanings to a lubrication engineer ('tribologist'), but it doesn't mean that the oil acts like some sort of 'cushion'.

Keeping in mind that very high gear tooth contact pressures and shearing forces exist even in moderatery loaded gearboxes (far in excess of those found in engines), are you suggesting that purpose formulated gear oils are typically lacking in film strength compared to engine oils?

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Originally Posted by e240

Unfortunately, most consumers tend to follow the advice of your so-called "Performance Specialist" who are only concerned about the symptoms, rather than the cause, and usually end up recommending big-brand (and often expensive) performance gear oils.

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Why would "your so-called "Performance Specialist"" be "only concerned about the symptoms, rather than the cause"?

Sure he (/she..) will want to sell you a product with a good profit margin, but he won't want to see you back in a few weeks blaming you for their broken gearbox...

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Originally Posted by e240

This is a 100% NO GO on a Honda Transmission. Although GL4 & GL5 Rated Gear Oils contain shock protection additives, they are also too thick for the oem Honda lubrication system.

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What "system"? It's all 'splash' and 'fling' feed. Any good quality liquid lubricant will get into all the nooks and crannies and lubricate adequately, nothing will 'run dry' due to the oil being a bit thicker, there is no oil pump to be damaged by the higher pumping loads created by a thicker oil.

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Originally Posted by e240

Lubrication is the key word here.

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And, compatability with the metallurgical and tribological requirements of the synchromesh rings, something that can be found with the correct grades of gearbox oils (i.e. good quality synchromesh compatible fluids).

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Originally Posted by e240

A basic analogy would be: Oil is your transmissions blood supply and needs to flow smoothly. Clog it up with cholesterol (i.e thick gear oil), and it will have a heart attack. Not enough Iron (i.e shock additives), and it will get anaemic/weak.

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An irrelevant analogy, but I agree that the correct additives should be formulated into the oil. I doubt that many engine oils have particularly high levels of "shock additive".

I'm not saying that thicker gear oils are inherantly better than thinner oils. Compared to thinner oils, thicker oils will have implications for fuel economy and parasitic power losses, and related to this may well tend to run hotter.

I am saying that I don't really agree with your critique of "performance gear oils" for the reasons you give.

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Originally Posted by JohnL

Can you detail this "comprehensive analysis"?

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We have been racing Honda's since the 90's, and have literally thousands of customers/acquaintances who we have been working with.

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To the best of my knowledge, for some years Honda has specified their MTF for all their manual gearboxes (and retro specified this oil for boxes that pre-date MTF availability), and this is a gear specific oil, not an engine oil.

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Pre-94, Honda Service Manuals state the use of Engine Oil. This is because MTF was not available yet. Honda MTF is basically just Engine Oil + Friction Modifiers.

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Why? State a case, don't just make an unsupported statement...

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State a case? The Honda Transmission.....

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Dog teeth engagement on the forward gears is used on nearly all gearboxes (I am aware of some Bugatti boxes that use sliding / meshing gear tooth engagement, similar to the usual reverse gear engagement, there are most likely others). This is the case for gearboxes with and without synchromesh, the only significant difference being that synchromesh boxes tend to use smaller and more dog teeth that engage in closer fitting 'dog slots'.

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Technically, the engagement teeth on both synchromesh and dog transmission are called Dog Teeth. This does not mean the engagement system is the same. Synchromesh transmissions use a synchronous engagement (i.e with the use of synchros). Dog transmission use a dog engagement (i.e no synchros). This is just what it is called, I did not invent the terminology. As I mentioned though, this point was irrelevant and nothing to do with the lubrication system ;)

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Please explain the mechanism by which this damage is created by virtue of the oil simply being thicker...

Why are you implying that all "high performance" gear oils are thick oils? This is far from being the case, the Motul gearbox oil in my car is quite light being comparable in viscosity to 10w30 engine oil (or maybe that's 15w40, can't recall offhand).

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Have you ever taken apart and inspected a Honda transmission before? Do you know how the lubrication system works in a Honda transmission?

I don't imply in my statement, I market i.e This is marketing material. Of course there are other transmission fluids on the market which may very well be as good as the Torco that we offer. Doesn't mean I'm going to recommend them. Just because a manufacturer tells you that you will make 20hp from their cams, doesn't mean you will ;)

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An issue may possibly arise if an oil that is formulated for use in a non synchromesh racing box is used in a gearbox that has synchromesh. If using a gear oil ("high performance" or otherwise) that is formulated to be compatible with the metallurgy of synchromesh rings and the frictional requirements of the synchromesh action, then there ought to be no problem...

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Published viscosity numbers are not the only "number" that is a variable in what type of fluid to choose. However, manufacturers rarely every publishes these other "numbers", unless specifically asked.

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"Hypoid transmissions"? The only place you will find 'hypoid' gearsets is the differential's crownwheel and pinion in rear wheel drive cars (and it would be ill-advised to use a hypoid oil in a gearbox).

In all gearbox applications (FWD, RWD, large, small), the gears will be 'helical' or 'straight' cut (with very rare and archaic exceptions, e.g. 'herringbone' cut). Even FWD differential crownwheels / pinions are not 'hypoid' (they are typically helical cut).

There is nothing inherantly "specialist" about Honda manual gearboxes, they are good quality but quite conventional boxes.

If you claim to be a gearbox specialist then you should know this basic stuff...

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Open up a Subaru transmission ;)

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And you are suggesting that engine oil (an oil only 'accidentally' suitable to use in a gearbox application) will be superior in 'shock' protection to the specifically formulated and Honda recommended MTF??

You need to be careful with the term 'shock'. In this context my understanding is that it really means the oils ability to withstand momentary and sustained extreme pressures and shear loadings without compromising the oil film and thus suffering metal to metal contact (i.e. 'film strength'). It may have other explicit or implied meanings to a lubrication engineer ('tribologist'), but it doesn't mean that the oil acts like some sort of 'cushion'.

Keeping in mind that very high gear tooth contact pressures and shearing forces exist even in moderatery loaded gearboxes (far in excess of those found in engines), are you suggesting that purpose formulated gear oils are typically lacking in film strength compared to engine oils?

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No, I never said engine oil was superior, hence why I said Honda MTF is actually ok to use. I'm sorry, but I don't know how you came to this conclusion from my statement?

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Why would "your so-called "Performance Specialist"" be "only concerned about the symptoms, rather than the cause"?

Sure he (/she..) will want to sell you a product with a good profit margin, but he won't want to see you back in a few weeks blaming you for their broken gearbox...

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Of course they wouldn't want to see you back, but the fact of the matter is, they do. Why? Because most performance specialists don't have a clue about how to maintain a transmission.

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What "system"? It's all 'splash' and 'fling' feed. Any good quality liquid lubricant will get into all the nooks and crannies and lubricate adequately, nothing will 'run dry' due to the oil being a bit thicker, there is no oil pump to be damaged by the higher pumping loads created by a thicker oil.

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Again, have you even opened up a Honda transmission before?

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And, compatability with the metallurgical and tribological requirements of the synchromesh rings, something that can be found with the correct grades of gearbox oils (i.e. good quality synchromesh compatible fluids).

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You'd be surprised at just now many fluids there are on the market that either 1) destroy yellow metals, or 2) decrease the functionality of the yellow metals (e.g friction modifiers). A lot of the times, this is not included in the manufacturers marketing materials.

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An irrelevant analogy, but I agree that the correct additives should be formulated into the oil. I doubt that many engine oils have particularly high levels of "shock additive".

I'm not saying that thicker gear oils are inherantly better than thinner oils. Compared to thinner oils, thicker oils will have implications for fuel economy and parasitic power losses, and related to this may well tend to run hotter.

I am saying that I don't really agree with your critique of "performance gear oils" for the reasons you give.

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Not irrelevant at all. You are correct, engine oils don't have the additives, but this does not change the fact that pre-94, Honda recommended and used Engine Oil in their transmissions and had designed it to be this way.

I won't bother posting any replies after this, as all it will do is start an internet war which is fruitless. Everyone is entitled to their own opinions, included yourself. However, your replies to our marketing material is misunderstood at best.

:beer: