I use Redline gear oils,they tend to seep slightly where a mineral gear oil might not in vintage trannys. 3 and 4 speeds like yours don't have oil seals, just oil slingers, I don't think the synthetic slings as well as regular gear oil.The oil may be seeping past the front retainer to case gasket or the tranny is overfilled slightly.Check the lube after a ride when it's warm and expanded in volume.
