In regards to valve seat recession, Pat is essentially correct. Since the use of unleaded gasoline fuel, the incidents of seat recession are few and far between. Remember the properties of propane, it has an octane of 104, meaning that in addition to other characteristics, the fuel has a long burn duration. If the ignition sequence is not initiated at the proper time, the combustion is still occurring when the exhaust valve opens, superheating the valve. This can cause tuliping of the valve, and once it breaks the valve seat seal, the seat can recede.
A properly set up engine, compression, valve timing, and ignition system, will show no adverse reaction to using propane as a motorfuel.
Franz