Which Oil is Thicker?

[outatime]

10w50 or 15w40?

I currently use ELF 10w50. However, I noticed the engine still consumes just a bit of oil... Not as much when I used to had Honda FEO... I'm thinking should I go to a thicker viscosity? is 15w40 thicker than 10w50?

cheers.

[JohnL]

These are 'multigrade' oils as opposed to 'single grade' oils. The first numbers (i.e. 10 and 15) are to do with the oil viscosity (thickness) when the oil is cold (some specified exact temp), and the second numbers (40 and 50) are to do with the viscosity when the oil is hot (also a sepcified temp). The 10w50 oil will have the same viscosity as a '10' single grade oil when cold, and the same viscosity as a '50' single grade when hot (etc). Single grade oils change viscosity with temp far more than do multi grade oils.

So, 10w50 is thinner (than 15w40) when cold and thicker when hot, which is good for a faster pump around when cold, and less oil consumption when hot (less oil passing by the rings and down the valve guides). However, the thicker hot viscosity creates more pumping losses (thicker = harder to pump) and drag on the pistons / rings / bores, so results in a slight loss of power at the flywheel and slightly worse fuel economy.

15w40 is thicker when cold and thinner when hot, meaning less pumping loss / oil drag when hot (than 10w50), so assuming good compression (poor compression changes the equation), will mean slightly more power and better economy, but possibly more oil loss.

It is of course a good thing for the oil to be thin when cold so that oil pumps quickly to the working surfaces (90% of engine wear occurs at start up, at least that's the commonly quoted figure). It's also good to use reasonably thin oil (when hot) for maximising power and economy, so long as the engine isn't consuming it at a fast rate.

One thing to note, the wider the gap between cold and hot viscosity numbers, the more additives will be used (typically) in the oil composition to achieve the breadth of the 'multigrade' rating, and the more additives that are in the oil the more the oil will tend to degrade over X time, or the more quickly the oil will tend to degrade to X degree. This is because many of the additives will be 'long chain polymers' (literally long strings of connected molecules) that are physically chopped up over time by shearing forces seen inside the engine. If using a wide range multigrade then in theory the oil should be changed more often...