Need help picking correct oil for my 25 year old 19hp twin Generac generator

[Texasmark]

I bought my 11kw, Generac built V twin in 2014. I run it on a direct line to a 500 gallon Propane tank. Once a year I change the filter and oil and use 10w-30 full synthetic. With the Propane fuel, the oil comes out about as clear as it was going in. I have had numerous outages and it runs a checkup run for 15 minutes once a week.

That's my story.