The Onan 4000 RV generator used 1.6 quarts of SAE30 or 15W-40 oil.
At low temps you can use 10W30 or 5w30.
You should change the oil every 100 hours or year.
You can do it yourself with:
t30 torx bit, 12mm socket.
Video clip how to change the oil:
Cummins Onan generator specs:
4-cycle, OHV, single horizontal cylinder
max power at 60 hz, Gasoline – 8.3 bhp (6.2 kw)
oil capacity 1.6 qt
Cylinder Crankcase: Aluminum alloy with iron
cylinder liner cast into the block, aluminum alloy oil base
Cylinder Head: Aluminum,valves in head
Fuel use: .7 Gallons per hour at full load