My problem is that naturally occuring vanadium in the fuel I use forms in tiny little beads on the valve seat. Vanadium is extremely hard and when the valve closes on it, these little pieces get driven into the seat. Then, they eventually fall out leaving a nice little pit. We also have problems with oxides of sulfer condensing out of the combustion gasses after all of the fuel has expanded within the cylinders. Well, oxides of sulfer and water from combustion mix forming sulfuric acid which eats away at the cylinder walls and decreases the time between overhaul. On our particular design of engine, the sulfuric acid can get into the lubricating oil where it eats away at bearings so we must use a more basic oil. Too basic of an oil will eat away at bearings too, so we must maintain a very delicate balance.