Walmart and surely other places, has 30W HD (High Detergent) Conventional (Dino) motor oil. I don't think you'll find 30W Synthetic. If you want to be ONAN Proud, get the name brand 30W and change on their schedule.
Full synthetic seems to be available in 5w-30, 10w-40, but not 15w-40 except in some diesel oils. That's OK, but do you really want to buy 5 gallons?
Our temps run a little lower, so I think full syn 10w-40.
Remember that owner's manuals don't morph to the latest info as oils improve and are tested. I found an owner's manual for 4KY up to Spec P, which has got to be pretty recent, and the oil chart was the same as above. I also checked to make sure we were looking at specs for the air-cooled 3600 rpm ONANs, and we were. I'd imagine their liquid cooled and diesel generators might have different specs.
