A mono 40 would work.
I have used countless gallons of rotella sae40 in a few onan genpack's over the years.
Any mono30 will treat you well.
A multi wieght has viscosity index in them, and will shear due to the fact that air cooled ope's run hot.
So I found Toro 30 grade full synthetic at ace hardware. The bottle says on the back 30/10w30. I'm guessing it performs like a multi viscosity? Well I got it and changed its oil out today.