Mobil 1 in everything - unless it is old, worn out or leaks.
Cheap insurance and the oil does not break down due to high temperatures until it exceeds 500 deg. F.
While I agree that the high temp characteristics of synthetics is a plus, especially in air cooled engines, you also have to consider that this is a flat tappet engine, which don't like the newest (SM, SN) oils. Reduced zinc and other antiscuff agents in the new oils is the problem. It is mostly in oils of X W 30 or lighter, as some, not all, of the thicker oils of SM rating still have adequate zinc, so 15-40 might be good, especially in diesel oils. The mentioned Walmart oil does have adequate antiscuff (have seen an analysis), so it should be fine. 5W30 Mobil1 does not have adequate additives for a flat tappet engine, so I would not want to use it without an additive.
I know this is controversial, especially with synthetic oil advocates, but even Amsoil has now come out with specific, high additive oils for older and flat tappet engines, because their newer oils were causing problems with them.
I've used dino Castrol in all my air-cooled engines going back to the days when I owned a '66 VW Baja Bug in the late 70's. Not a single lubrication problem so far. I select the weight based on the expected temperature range and the chart in the Onan owner's manual, and recommend that others do so, too. It's really easy to do, and I'm sure Onan has put a lot of research into developing those charts. They'll be picking up the tab if if you have an engine issue under warranty, if you follow their recommendations.
Jim, "Bad typists of the world.... Untie!"
'06 Tiger CX 'C Minus' on a Silverado 2500HD 4x4, 8.1 & Allison (aka 'Loafer's Glory') www.tigervehicles.com