Actually, the ashless oils were designed for water-cooled outboard engines. The low-ash oils were designed for air-cooled engines. The description of ashless and low-ashless simply refers to the results of a combustion test. The detergent in the ashless oils leaves no ash when they cook the oil at high temperature. The low-ash oils have a detergent that leaves a little ash when they cook the oil. The ashless detergent works best in low temperature water cooled boat engines. The low-ash detergent works best in hotter running air-cooled motors. Your best bet for our engines is to run a low-ash oil. Some oils are designated as "all-purpose", but they are a compromise by design. The confusion comes in when some manufacturers of model engines specify an ashless oil (like Lawnboy) for break-in. I would rather use Penzoil two stroke oil for air-cooled engines (low-ash).