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Oil viscosity is dictated by Cat but there is usually a range depending on where you are operating your coach, more importantly what temperatures you will be operating it in. Generally most run 15w40 unless you do alot of northern driving in the winter then you could go with 5w40. Really the 5w40 and 15w40 give you the same hot temperature protection, the lower number is for winter operation, so the 5w is going to give you a lower viscosity to help with running in cold temps. 

One thing to remember, oil always gets thinner as it get hotter. Oil additive packages though can help with reducing wear even though the oil gets thinner, this is where the real science is working to protect engines better these days.

If you want to know more about oil than you ever thought about, this guy gives a pretty good run down all about oil:
https://www.youtube.com/live/3yfTEU7xTFo...t_jvpPUQXA