I use Rotella T4 15W-40 diesel oil. It is on the approved list for my Onan 5500 and since it is the same oil I use in my truck I only have to keep one type of oil on hand. 188,000 on the truck and 470 hours on the Onan and neither one has ever used any oil.
2006 GMC 2500HD CCSB 4x4 Duramax/Allison, Titan 52 gallon fuel tank, Prodigy Controller, B&W Companion Hitch
2010 Jayco Designer 35RLTS, Cummins/Onan RV QG 5500 EVAP, Progressive Industries EMS-PT50X, TST Systems 507 TPMS, RV Flex Armor Roof