Not saying I would or reccomend doing 15 to 20,0000 mile OCIs with M1 oils, but a guy I worked with has done that very thing in his 99 Silverado (M1 10-30) and the engine still runs great with 250,000 miles. I also know he has never done a UOA so if he did it might scare the wiz out of you. Now it would be interesting to see how EP would hold up with those miles, but ima
not doin it. I'm a 10,000 mile OCI guy no matter what super duper oil comes along.