I have been using Shell Rotella T6 Full Synthetic Diesel Motor Oil SAE 15W-40 in my Onan QG4000 now for over a year and no issues. I live in WA state, so temps range from 0F-110F.
I dont carry extra oil as I am afraid of a spill in my RV but i do check the level prior to long trips in the generator as well as the RV engine.
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