Originally Posted By: tig1
Why do you guys hate XM? I hear you say it but no explaination.
Years ago I was an M1 fanboy. Then I got caught up in the whole basestock debacle,then the castrol hearings and I just swung over to the Amsoil bench.
And if you like a Amsoil you naturally have to hate Mobil.
Then I tried it out for old times sake and my engines got very noisy and I didn't like that they cheapened the basestocks after the castrol thing but the cost to the consumer didn't change so I just decided that's it,no more Mobil.
I'll buy the stuff when its cost effective and on sale but I won't go out of my way to search got it.
Originally Posted By: Gokhan
Elastohydrodynamic-lubrication (EHL) film thickness is given by (
reference):
Here H_c is the EHL film thickness, alpha is the pressure - viscosity coefficient, and eta_0 is the high-shear (HTHS) viscosity.
The goal is to have the pressure - viscosity coefficient as high as possible, as well as the HTHS viscosity, to achieve a higher EHL film thickness.
Unfortunately, Group IV (PAO) has the smallest pressure - viscosity coefficient, followed by Group III, which is followed by Group II. Group I has the largest pressure - viscosity coefficient. Here is why (from page 180 of the
reference):
Therefore, synthetic oils will give smaller oil film thickness than Group II oils and Group II oils will give smaller oil film thickness than Group I oils in the EHL regime. EHL regime and boundary-lubrication regime both apply to the valvetrain.
One advantage of synthetic oils is that, due to their use of lower concentration of viscosity-index improvers, their HTHS viscosity will shear less than for conventional oils over the OCI. Higher HTHS viscosity is good for oil-film thickness. Therefore, synthetic oils will maintain their oil-film thickness better than conventional oils over the OCI.
After all this said, am I worried about using synthetic oil? No, I am not. I believe the most important protection against wear in the valvetrain is ZDDP, moly, and other antiwear additives. So, if you have a decent concentration of quality (mostly secondary kind) ZDDP, the trinuclear kind of moly, and some additional quality AW/EP/FM additives, you should have good wear protection in the valvetrain regardless of the viscosity or the pressure - viscosity coefficient. This is not a proven statement though but more my current understanding. Engine tests are always the most authoritative in reaching a conclusion.
So this explains why conventional oils tend to make an engine very quiet,because the oil film is thicker but not necessarily stronger.
Am I to infer that yes a conventional oil has a thicker oil film at any given temp however the film strength is lesser than their synthetic counterparts?
Thanks in advance for any answers if you know them.