If OCIs are keep to about 5000 miles I believe dino will run an engine way down the road, but if the owner doesn't abide by that, as my friend did with his late 80s Mustang, massive sludge can form and nix the engine. My friend drove a lot of miles on his job,50,000 miles+ a year. He simply overlooked his OCs, at times 12-15,000 miles, and cooked the engine with 130,000 miles in Maryland on the interstate. Lost the oil pressure, had it towed to a garage where they discovered the heads and pan were filled with sludge. New engine was installed and thereafter he went with synt. oil. He retired the Mustang with 500,000 miles total to his garage where someday he plans to restore it.