The "hardest" use of an engine, thus requiring more frequent oil changes is stop and go driving in high heat with the A/C running... OR short trips of less than 10 miles, especially in winter cold.
Especially with standard oil (vs full synthetic) as the high heat, piston ring wear (blow-by) and moisture can break down both the petroleum based oil and the viscosity additives (thixotropic additive). With full synthetic oil, the breakdown is significantly lowered, but not eliminated and moisture contamination is the same for both. In addition, petroleum based oils with a high paraffin content are better at not draining off the metal surfaces when you shut the engine off. However, even without high paraffin content, most modern petroleum based oils reach full pressure and metal surface coating within a few seconds of the engine being started.
Almost none of the above applies to typical RV use. Most RV's are no used as commuter vehicles, they start, drive short distance at "city" speed limits and then 100's of miles at highway speeds. Then they sit, for days, weeks or months never being started.
In other words, there is no need for any extra oil supplements. This is especially true for engines that use full synthetic oil exclusively.
I just got back from a 5,000 mile trip. The PC now has 60,000 miles on it. The engine oil is about 8,000 miles from its last oil change. I checked the oil a few times on that trip, always at the full mark on the dipstick, light brown in color. I have not, and will not used oil additives in close to 40 years and have never had a lubrication or engine wear issue.
The only thing I do "different" is I use 5W/30 full synthetic vs Ford's recommended 5W/20 oil. I like the thicker viscosity at the higher temps.
So, all of the above means, in my humble opinion and experience, oil additives are not needed and are a waste of money. Your mileage, literally, may vary.
Don
