If you own a four cycle engine please don't read this post. There's no information here for you and I don't want an argument, thanks for your consideration. :)
If you own a two cycle Detroit Diesel and you follow Detroit's instructions it is possible to greatly extend your oil change intervals and save a lot of money. You have to use the oil and filter that Detroit recommends, drive long distance highway miles with no off road driving, and change your filter at least every 25,000 miles. Chevron Delo 100 straight 40 weight motor oil is what I've been using, along with Detroit's 23530407 oil filter. At 25,000 miles I changed my oil filter and took an oil sample from the center of the old oil filter with the oil sample kit I got at the local Detroit dealer. The sample bottle fits right into the center hole of the filter perfectly, then tilt the filter to fill the bottle with oil. The sample kit is prepaid for any of Detroit's laboratories. They have several labs around the country. You have to call them before you mail in your first sample to set up an account. Detroit's lab will then email the results back to you in a few weeks. I just got my lab results back today... (please see attachment)... As you can see they said the oil sample was normal but the viscosity was off, probably because they thought it was 30 weight oil, but I'm sure I wrote down 40 weight on the sample. I'll attempt to correct that error with my next sample. You don't really have to send in an oil sample to Detroit's lab in order to do this extended oil change interval. So there you have it. If you own a two cycle engine and you drive a lot of highway miles like I do you can save some serious money on oil changes. I was talking to a guy who owns a diesel engine machine shop that rebuilds Cummins engines. He owns a coach with a Cummins and said he has to change his oil every 15,000 miles according to Cummins. I presume that would cost at least $200 for each oil change and it would be about once a year for the amount of traveling I do. I have to add a gallon of oil when I change my filter. The filter costs $20 at the Detroit dealer and the gallon of oil costs about the same. The filter change is super easy and not messy at all. It's literally a five minute job. If you can get on a creeper and roll under your coach you can do a filter change. You don't have to warm up the engine if you're just changing the filter and not the oil. I may have to change my oil at 100,000 miles, or I may be able to go longer, I presume depending on the lab results at each filter change. It'll be several years before I get to that point even with as much traveling as I do. If you're considering purchasing a coach with a two cycle engine and you'll be doing a lot of highway driving don't forget to consider the cost and frequency of oil changes.
The two cycle Detroit Diesel engine can save you a bunch of money in many ways. The purchase price of a coach with a two cycle Detroit is usally much lower because so many people hate it. Most of these people are clueless. This works to your advantage tremendously as a purchaser. Throughout my life I've almost always found a huge advantage in doing the opposite of whatever the "herd" is doing. If you buy a two cycle Detroit you'll have the finest engine ever put in a motor coach, by far, and your only disadvantage will be cold weather starting. Read my post on cold weather battery modification to see what I did about that. Not that you'll live long enough to wear out one of these engines, but if you do, the two cycle Detroit will save you a huge amount of money on an engine overhaul. The parts manager at the Detroit dealer told me the overhaul kit for my 6V92TA costs $2,400 and the engine can be rebuilt in the chassis, saving a lot of labor cost. He said he's never seen one cost more than $5,000 to overhaul with everything included. He's been with Detroit since 1971. The Cummins machine shop owner told me his Cummins would cost up to $30,000 to overhaul at his shop. I could replace my entire coach for less than that, and I've got a way better coach than he does, just saying.
Even without driving high mileage, I found I could go years between oil changes with my 8V92 Wanderlodge using Delo 100 straight 40 weight. Sent an oil sample out for analysis every year, and always within acceptable limits. Eventually I would change the oil just because. Great engines.
I suspect an important part of the extended oil change interval is the very high quality of the straight 40 weight Delo 100 motor oil and the fact that the two cycle engine burns a gallon about every 2,300 miles, which means there's a gallon of new oil being added to replace the gallon that was just burned off. I never appreciated the superior quality of straight weight motor oil until I got this two cycle engine. I'm thinking it's probably possible to never change the oil in a two cycle engine if you drive long distances on the highway. My friend used to work for the railroad and he said they never changed the oil in their two cycle locomotive engines, but they did change the oil filters... while they were rolling down the tracks!