Let me paraphrase a respected mechanic I know who runs a big Cummins marine diesel shop in California: "I have never seen a properly maintained diesel engine harmed by regular, petroleum based lube oil."
Yes some synthetics have better lubricity properties than petroleum oils. And they last longer due to their chemistry resisting breakdown due to heat and time.
But they work well enough with petroleum based oil if you stick to Onan's oil change interval. If you want to extend it to 300 hours, then use synthetic oil.
David
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