Over here there are still a lot of vehicles converted to LPG (propane) - petrol is over $6 US gallon equivalent here; LPG is about half the price. There were conversions done in the late 70's & into the 80's that did have a valve recession problem; but with hardened seats fitted, they were fine. I think all vehicles today have hardened valve seats anyhow.
I've been told that there was also a problem on earlier conversions where certain alloys in the bearing inserts would be eaten away by the LPG but apparently this is no problem today either.
