Originally Posted By: 440Magnum
10w30 is NOT THICKER than 0w30 at 100C (212F) where the specification benchmark is done. As the temperature goes higher than that, the 0w30 may actually retain viscosity better because it will have a higher VI (and is typically a better base oil).
You may be right about the add pack, but frankly everything I've been reading lately indicates that the 0w30s and 5w20s are some of the best oils out there. Better base oils, good addpacks. Great UOAs...
You might also consider an HDEO in 5w40, like Rotella T syn, Mobil 1 TDT, etc.
It is not correct to say that 10W30 is or is not thicker than 0W30 at 100C. That totally depends on the properties of an individual oil.
Also, there seems to be confusion about VII's and that a multi-grade oil may actually be thicker than a straight grade above 100C. This may or may NOT be true, again, depending on the particular oils involved. It is not correct to generalize here.
Yes, a multi-grade oil such as a 0W30 will have a much higher viscosity index than a straight 30. However, viscosity index is only measured by using two points, 40C and 100C. This is the exact area where a multi-grade oil will behave as non-newtonian. This means that viscosity index is really a misnomer for multi-grade oils above 100C and below 40C. It is not correct to use viscosity index to extrapolate viscosities on multi-grade oils above 100C or below 40C because we don't know where those oils begin to behave as newtonian again. The VII's in those oils are only effective to reduce thinning between a minimum and maximum temperature. Once they are fully uncoiled, they do not add any further apparent resistance to thinning as temps increase.
That said, I agree with BuickGN. Go with a stouter oil for towing and severe use. Just to qualify: The 0W30 will work fine, but another oil may work better. Save the 0W30 for the winter. In any case, it will not go to waste. Engine protection is complicated, with many many parameters to account for, and thus does not only hinge on viscosity at 100C!