Oil changing with Engine Flush?

[aaronng]

If you are using a full synthetic, then the engine oil's viscosity is almost the same whether it is cold or hot.

[NogZ]

If you are using a full synthetic, then the engine oil's viscosity is almost the same whether it is cold or hot.

Just wanted to clarify things

The ratings 10W30 determine the viscosity of the oil when its cold and hot.
First number (10) is viscosity rating when the engine is cold, (30) is when its hot. Higher rating = thicker

Lower viscosity is used on cold so that on start up the oil is thinner and flows easily. Once at operating temp (and thus highre visscosity) the oil will stay on the cylinder walls, etc etc and do its job.

Synthetic oils are only different to Organic/SemiSyn oils in that they have higher resistance to breakdown caused by heat and it flows better in extreme cold. Better for performance engines, bit overboard on normal engines.

Nogz

[aaronng]

Just wanted to clarify things

The ratings 10W30 determine the viscosity of the oil when its cold and hot.
First number (10) is viscosity rating when the engine is cold, (30) is when its hot. Higher rating = thicker

Lower viscosity is used on cold so that on start up the oil is thinner and flows easily. Once at operating temp (and thus highre visscosity) the oil will stay on the cylinder walls, etc etc and do its job.

Synthetic oils are only different to Organic/SemiSyn oils in that they have higher resistance to breakdown caused by heat and it flows better in extreme cold. Better for performance engines, bit overboard on normal engines.

Nogz

Actually, the first and 2nd numbers are not equivalent. Going by your explanation, then oil at higher temperature is thicker than oil at a lower temperature?

For a 10w-30 oil, the first number is the equivalent viscosity of a standard SAE monograde oil measured at 0 degrees C. So at 0 degrees C, the 10w-30 oil has the same viscosity as an SAE10 oil at that same temperature.

The second number is for 100 degrees C. Same thing applies, the SAE monograde oil is measured at 100 degrees C and thus the 10w-30 oil has the same viscosity as an SAE30 oil.

You can compare cold with cold and hot with hot. You cannot compare hot with cold because they are not in units of viscosity.

So you can compare a 10w-30 and a 5w-40 oil and say that at 0 degrees the 10w-30 oil is more viscous than the 5w-40 oil and at 100 degrees C it is the other way around.

And the w means that 5W is tested at -25C, 10W at -20C, 15W at -15C and 20W at -10C.

[NogZ]

ahhh yep good point on the fact that you cant compare the cold value of an oil to another oil's hot value.

But with regards to the numbers (10, 30) not being units of viscosity, they are saybolts measurements (the amount of elapsed time in seconds x mL of the oil flows through a defined 'hole)

So in that sense you can compare the 10 and 30 because they are real measurements, but at different temps (you cant have 2 viscosity ratings for an oil with the same test variables) . Which then points to the fact that oil on start up is less viscous and then increases in viscosity (thicker) at 100C.

Not what you would expect as normally matter has less viscosity through heat. I read that it is acheived through polymers added to the oil, that are coiled up when colder, then unwinds into long chains through heat.

Yep W defines the viscosity rating in the colder temp (i read -18C for all 5-20 values, but its similar to the temps u brought up anyway)

[aaronng]

The Saybolt Furol Viscosity measurement is actually used for transmission fluid, gear and heavy fuel oils. I don't think they use it for engine oils, as the Saybolt method is under ASTM D 88, which is not in the list of engine oil tests below.

There are 4 tests done to classify the engine oils to SAE ratings.
1) Kinematic viscosity, ASTM D 445
2) High-temperature high-shear-rate (HTHS) viscosity, ASTM D 4683
3) Cold cranking simulator viscosity, ASTM D 5293
4) Cold pumpability, ASTM D 4684

The numbers still do not have units. They are just a rating given to oils of a particular viscosity. They are arbitrary numbers.

Here's the table as to how the SAE classifies the oil ratings based on the above 4 tests.


And from the same source, this is how they determine if the oil gets the W rating:

The table shows that there are two types of SAE viscosity grades: those with a "W," and those without. Those with a "W" have upper limits on low-temperature cranking and pumping viscosities, and a lower limit on kinematic viscosity. Those without the W have upper and lower limits on kinematic viscosity at 100°C and HTHS at 150° C, but have no low-temperature viscosity limits. So, W grades pertain largely to low-temperature performance (think of the W as meaning "winter"), while non-W grades relate only to high-temperature performance.

For the W grades, note that the pumping limits are specified at 5° C below the cranking limits. This is done to ensure that if an oil allows an engine to start, the oil will pump throughout the engine, as well. If this were not the case, severe engine damage could result.

[NogZ]

s the Saybolt method is under ASTM D 88, which is not in the list of engine oil tests below.

ur right I should've cross-ref

and of course since the 10W30 are not measurements, yes ur right they're just ratings for classifications.

thanks for the clarification