I also assumed they probably cranked up the detergents, but I don't know for sure. I figured if the car ran OK on regular oil, wasn't sludged up and didn't leak (and I fixed leaks when it did) then there was no need for anything other than the regular oil I always ran.
The cars I kept for the highest mileage were the ones I ran whatever brand of oil (but always the same weight) and what ever oil filter was cheapest. The ones with the most miles when I sold them were a 1988 Gran Fury (200k but rust was getting to it, as were suspension issues), 1995 Neon (225k miles, sold to a co-worker and it's still her backup car though it sees little use now) and a 1993 Dakota (sold with 217k to my neighbor who has been driving it daily the last 3 years).