Eric, you're probably right, the higher viscosity of the 15w-50w is likely making up for poor compression if it's that noticable a difference.
Most likely the rings. The old test for that used to be to do a compression test and if it was below spec.'s then pour an ounce of oil into the cylinder through the spark plug hole and redo the test. If that brought compression up to spec. then it would be rings/cylinder wall. If not then the problem would be in the head.
Yes, Mobil does make a 15w-50w Mobil 1. If the motor is working fine then 15w-50w oil is really only necessary for extreme heat conditions such as racing, especially with forced induction.
Regards, Alan Coles
