I recently switched from Mobil 1 0-30 to their Euro formula 0-40 that my local parts guy can get.
After reading all the oil specs and many comments/web sites on this topic, I find the hot viscosity (100 deg C)of 10-30 and 5-30 is just about at the SAE 20 spec. This is THIN ! This is about 17% of the viscosity at 40 deg C.
Notice how the viscosity varies far more with temp between 100 and 212 degress F than between 20,30,40,50 SAE weights at a constant 212 degrees F. so I can't get to nervous about "too thick oil". My previous experience with long life motors and the use of different SAE viscosity grades leads me to believe that the car manufacturers are sacrificing engine life for a minor boost in corporate CAFE specs.
