Originally Posted By: Audi Junkie
Originally Posted By: CATERHAM
Widman's viscosity graphs are of course 100% kinematically based and therefore prone to inaccuracy when comparing oils of markedly different base oil chemistry and VII levels.
Keep in mind it is still just an extrapolation based on two set points and has always been a problem when you get much below freezing. Now if we could figure something out between the HTHS vis at 150C in cP and MRV at -35C in cP or CCS at -40C in cP, now we would really have something.
You know that CCS and MRV are the least accurately reported specs there are, as many mfgs, like Castrol, simply list the spec minimum and other mfgs don't list any data at all.
Lacking a ~theoretically~ more accurate way to measure cold visc, I'll base my analysis on the crude extrapolation method. Seeing how most base oils and add packs are similar anyway, FOR COMPARATIVE PURPOSES, the visc calc is reliable. That is, it may not deliver a singly accurate number for any given oil, but it can easily project and compare a few products.
Anyone want to bet against PP 5w-30 being thinner than M1 0w-30 from 0c to 0f?
I didn't think so.
Much like the HTHS vis trumps the 100C k'vis spec', the MRV and CCS spec's are much more useful than PP.
Between 0C and 0F(-18C) I'd put my money on AFE being thinner at 0F although I think it's a rather pedantic discussion, as I don't think there's much between these two oils.
About the only practical way of knowing would be to test in a vehicle with an oil pressure gauge and an oil pump that doesn't go into by-pass at those cold temps'.