Specifically, ZDDP or zinc. Modern motor oils lack it because it clogs catalytic converters. I use high-zinc oils and/or a zinc additive in all my small 4 stroke engines. STP Oil Treatment is one additive that is inexpensive, contains relatively high zinc levels and has been around a long time. It would probably work well as an additive in 2 smokers, too.Similar to one of the latest Honda outboard service notices they actually recommend the older lower grade oils for their 4 stroke outboards. Newer oils have striped out some of the more lubricating properties. For the sake of car emissions newer oils lack these properties.