As long as the oil does not have friction modafiers in it then it is fine for use in the tranny, Dont go and use something like royal purple as it has lots. The only advantage to running synthetic is is is capable of withstanding more heat then conventional oils
You may find that the very hot running Honda air cooled "Radial four valve combustion" (or is it chamber?) really do well on M1 red cap 15-50. The camshafts in those engines are subject to serious stress and overheated oil will result in early camshaft and follower wear. In addtion, the piston ring packs seem to be prone to overheating in certain conditions. When this happens, most of the ring tension is lost and the engines smoke badly, especially on startup. The M1 will not cause a healthy clutch to have problems.