the new mustang engines are spec'd at .0003".0005" bearing clearance, gas mileage is the biggest reason they use the thin oil to begin with, the clearances are just a concession to the reality of 0 weight oil. My wifes car spec'd 5W-20, I ran a whole case of 10W-30 through it, didn't seem to hurt it any, it went 190,000 miles running strong. but the transmission died, and there was a very expensive feeling problem with the anti-lock brakes, but the engine was golden.
