I've known others who have used that oil, and the weight in their gensets with no issue.
I personally never go below 15w-40 in any of my equipment, using synthetic or dyno.
I base this on the heat range the genset will be producing as it works.
I know the weight you plan to use in a synthetic works well in a high heat range, but its just a little out of my comfort range being a little old school.
You would be well advised to get an official answer from a Cummins Onan repair shop.
Please let us know what they say....
Dan & Loretta, US Army Retired Aero Scout Pilot
2012 Fleetwood Providence 42P Class A/DP
Spartan Chassis, 8.9L Cummins 450HP