OK, I have been trying to follow the various oil controversies: the latest is that 10-40 is BAD and new car makers are warning not to use it (we should use 10W-30).... the reason?
10W-40 has more polymer viscosity enhancers to make the oil shift 30 points from 10W (cold) to 40W (hot), compared to 10W-30 that only shifts 20 points. The oil is the lubricant (not the viscosity enhancers) so you want the one with less of the viscosity enhancers...
My Chevy Cavalier says to use 5W-30, which is "25 point shift". Is there really a significant difference in the volume of oil in comparing 10W-30 and 10W-40 or 5W-30?
My owners manual recommeds the 5W-30 because it's operating range allows outside temps down to about -20F. 10W-30 or 10W-40 goes down to about 0 degrees F. Not that I care living in california.
Anyway: what % of the volume of a quart of "oil" is made up of the viscosity polymers?
Why is it every quart of oil I have in my garage which are: full synth (10W-30), synth blend (10W-30), Castrol/valvoline conventional oil 10W-40, all have the same API service rating of "SM" which I believe is their highest rating?
Are new car makers actually saying that oils rated at the highest service rating are not good enough?