Originally Posted By: pkunk
My old '81 F250 straight 6 on propane got a yearly oil change regardless of miles and the oil was light amber. Never used any special oil, just what was the cheapest. Still that way after 350K miles and new owner. He can't believe it.
Gaseous fuel burns without soot (Propane is stored as a liquid, but you have a heated regulator and a diaphragm-type carburetor to turn it into a gas), so you won't get darkening of the oil the same way you would from liquid fuels.
My instructor for the LPG vehicle course harped on the fact that oil won't get dark and needs to be changed as the additives wear out, that spark plugs tend to need to be one or two steps cooler to last, run better, and get better power, and that based on tailpipe tests a badly tuned propane engine pollutes much more than a gas engine. A well tuned one pollutes a whole lot less. Oh, and to run a much cooler thermostat, since propane engines care less about engine temperature, nor do their emissions because of it.
And yes, with proper maintenance, they run a really long time. You're not burning a solvent that dilutes the oil coat on your cylinder walls, for starters. And engines specifically built for the fuel can be super awesome, I remember being able to lay a strip in a 3-ton 30' panel van with a 4-on-the-floor and dualies pretty much at will. I'd run that fuel today if I could.