"Jeff Khoury", DsrtEgl, said ...
> The first number refers to the cold pumping viscosity, the lower the
> better. A 0w-40 and a 15w-40 have the same viscosity at operating
> temperature.
+1 on the above. The only clarification I'll offer is that the "operating" measurement is made
at 100C, and engine oil often exceeds that "some". The viscosity curves MAY not track
exactly ... but we usually ignore that minor difference.
> It will make no difference, in fact... they're better for your engine
> because they pump better at startup, which is where most wear happens.
I disagree a little here. If the 0W50 remained 0W50 through its life, there is absolutely no
argument that it would be excellent. According to most experts, the issue is that the wider
the spread in viscosities at reference temperatures, the shorter the time until the products
"shear down".
This shearing seems to occur more quickly in gear systems than in engine only systems, in
other words, common transmission/engine sump systems are more "damaging" to multi-
viscosity oils than engine only.
I'm one who likes the 5W40 Rotella T6 ... in the winter. I ride about 4000 miles a year, and
use "regular" 15W40 Rotella in my spring change, the synthetic in my fall change. In
particular, I find the cold cranking superior with the 5W fluid ... although not by much. In my
OPINION, the superior pumpability (is that a word?) helps the first 1 or 2 minutes as well.
I find that the shifting with the T6 near EOL (1500-2000 miles) is poorer than with the regular
at its EOL (2000-2500 miles).
Just my 2 cents worth ... I'm a hydraulics guy with lots of oil experience.