Factory Onan tag on mine (Ingersoll) says above 32F either straight 30 or 40 weight
If it`s hot in OK this year ( and I assume it is) I`d go 40
If you have high hours and oil consumption issues I`d step up to 50 weight.
With 2000 hrs and temps in the 90`s, mine is much happier with 50 weight, which the Ingy manual recommends if the lowest expected temp is 60 degrees. I don`t expect it will be under 60 for a while ... If you warm an engine up gently I would anticipate no problems with heavy oil in the summer.
Yep what PA 318 said,these engines have horizonal valve guides and light multi grade oil tends to be SUCKED out through them and smokes.The valves require adjusting every 200 hrs but not many get it done.