flyerdan
Veteran Member
Chances are that the main bearings are worn enough that it bleeds oil pressure at idle below the 5 or 10 psi threshold of the switch for the light, as you've found upping the idle a bit gives enough psi boost to keep it off. Going to a thicker oil would help, but depending on climate could cause winter issues with starting hard. Replacing the main bearings is an option, but not typically a first timer project.