Best synthetic oil for a 1984 Honda V65 Magna?

You can use any 15-40 W motorcycle engine oil that is JASO-MA or MA2 approved, similarly you can use all SAE SF and SG approved oils. As the bike is a 1984 model I would check if the engine was much worn and in that case go for a single grade 40 W oil of the specifications mentioned. Fully synthetic or semi-synthetic is immaterial as long as the oil is approved by the required standards.