VCCA Home Forums TECHNICAL - GENERAL Mechanical - Engine, Trans, etc multi weight oil?

OK, I know this oil question stuff has been discussed to no end!
However I have not found the answer to my question. I under stand that multi weight changes viscosity with temperature change, right? 10W30. At say, 50/60F this would be the same as 10W, right? At what temp F would it become 20W and what temp would it become 30W??

This might help some. I would pay special attention to the section with the header "5 W is not 5 Weight".

Thanks Tiny, I think. Man-O-man do I feel stupid now. I had this oil thing ALL wrong!!! Are you sure this guy has all his facts correct? 300 pages about oil.

Anyway, thanks again.

I will try to not be too technical so people can understand.

Actually the guy is only partly correct. Multi-vis oils contain viscosity index (VI) additive(s). These are polymers that dissolve in mineral oil (base oil) and have increasing affect on the viscosity of the mixture as temperature is increased. Basically they are more linear at lower temperatures and can slide past other polymer molecules relatively easily. As temperature increases the polymer molecules act more three dimensional and therefore reduce the flow-ability (increase viscosity) of the mixture when compared to an oil without the additive.

That is a good simple description except for real synthetics that are actually thicker oils designed to not thicken up at low temperatures.

I have about 3000 pages on oils at http://www.widman.biz (some in English)

Note: Contrary to common myths, single grade oils usually have higher consumption than a good multigrade, since the single grades don't clean off the cylinder walls as easily on the down stroke.

Not 100%. I've heard both ways but his solution is pretty straight forward. I plan to do a garage test by taking two equal quantities of a multi grade oil and testing the viscosity myself. I'll test one sample at room temp and the other at the prescribed 100c while timing both. If the high temp is more viscus it'll be the first time I will have seen a liquid get thicker as it warms.

The oil does not get thicker as it warms but does not thin out as fast as a single grade oil. Though not exactly correct the common description of a 10W-30 for example, having a viscosity of a 10 weight at freezing temperatures and of a 30 weight at boiling water temperature at least gives a good idea of what happens.

Because the synthetic oil base stock generally has a lower surface tension than mineral oils it does have a tendency to leak more. It also tends to swell some elastomeric (rubber) compounds more than mineral oil, so leakage past seals can be reduced while past metal to metal parts (like piston rings) can increase. Because the synthetics have more surfactancy they also tend to clean deposits from metal surfaces. That can initially increase oil consumption and later reduce it.