Originally Posted by dsc
That recommendation makes no sense to me because 5W-30 and 10W-30 are supposed to act the same way at high temps. The difference between those oils is at low temps. Isn't it?
The SAE has a range for each oil viscosity. This Advance Auto Parts
site has good info.
"SAE 30 is SAE 30 no matter what the "W" prefix number is: 0W, 5W or 10W. This viscosity in centistokes (cSt) @ 100 degrees C is with the minimum of 9.3 cSt and a maximum of 12.5 cSt."
You can have two 5W30's with the thickness of one on the lower end of the range and the other on the higher end. Generally speaking, the one on the higher end will probably give you more protection. For example, M1 10W30 is 10.0 Cst @ 100C and German Syntec 0W30 is around 12.3 (IIRC). You're probably going to get better protection from the Syntec (then there's additives, base stocks, etc.).
You would think that all 30 weights would be the same, but that's not the case.