The oil in air cooled engines is part of the cooling system. Synthetic oil does not absorb heat the way regular oil does, also engines with the splash oil systems (no oil filter) should use a non detergent oil, as it does not foam when churned up by the moving engine parts. Non detergent oil come in straight weights like sae 30, 40, and 50. the engines with oil filters can use regular detergent multi grade oil like 10-30