copy and pasted from Got Propanes FAQ
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"Will propane hurt my engine?
NO! In fact, propane is on the light end of the hydrocarbon scale, meaning there is almost no carbon in it. Carbon is what turns your motor oil black. As a result of using a propane system, your oil will stay clean in a new engine and will get cleaner each time on an existing engine. Because propane goes into the engine as a vapor, there is no "raw" fuel entering the cylinders, which washes past the piston rings and into the oil. This is a major reason propane engines last about three times as long as a gasoline engine.
Propane will not burn the valves. On older engines withour hardened valve seats,this used to be a problem.Even this wasn't a problem until tons of miles were accumulated.On engines designed to run unleaded-no worries."
