I have a fuzzy memory about either propane or NG being hard on the engines. I think it was NG.
My understanding is this:
Automotive gasoline includes anti-wear additives as part of the mix (not lead anymore, but replacement compounds).
NG and propane don't have these. They burn very, very clean. So, in a converted gasoline engine, it's critical to change to an oil that is rated for NG/propane use. These provide the extra lubrication necessary.