My 1989 has had Valvoline 10W30 engine oil in it since new. I added magnets from Radio Shack to the drain plug after seeing sparkles in the oil. It still sounds pretty good and has 224,900 miles on it. About 100,000 of these had either a 7700 lb travel trailer, an eight foot slide in full height camper, or both aboard. It is a little noisier than when new, but not much. The engine oil works well. I never saw a reason to question the design engineers.