Prior to unleaded gas, the tetraethyl lead anti-knock additive also "cushioned" the valve from the seat. When they phased out these types of additives, we were left with unleaded gas. Without this cushion, the valves would recess the valve seat leading to ruined heads and burned valves. Some engines have replaceable valve seats (even a briggs and stratton) but most have the seat cut into the head. One fix for this was the use of a stellite torch to hard face the valve seats, then do a valve job, lots of harleys had this done. Post Unleaded engines have their metalurgy adjusted for this difference. LPG and LNG do not have any lube characteristics for valves. BUT, I have seen IHC binders run on that LPG for a lifetime (100+k miles) Incidentally having 20k mileage oil that looked as clean as when it was put in, although its lube characteristics had to be diminished. Put a properly designed LPG or LNG system on a modern engine and you will have a sweet clean runner. Both have less BTUs per unit than gas so power will suffer. On an older truck, put on a kit designed for the approximate CID and drive it. Heads are cheaper than stellite or inserts, find an extra head, IF you ever have a problem, put it on.
