I had a 1980 ford F-350 with a 351 V8 on Propane since it was new.
I only changed the oil on that thing about ever 25K miles. We drove it to 380K miles and sold it. It's still in the neighborhood, I see it sometimes. I have had other vehicles on propane gas boy do they run clean.
The ford I talked about above made it to about 375K miles before it broke a valve spring. I took the valve cover off for the first time and stuck a chevy valve spring in there, it's still running today. The top of the head was spotless after 375k MILES WITH 25k OIL CHANGES ON lp GAS.