FYI 20w50 Castrol is not as good as the other products mentioned.
The Spectro and Bel Ray have gear specific polymers and extreme pressure additives as well in their oil to cushion the gear faces and to protect the sliding surfaces of the tranny. They are also clutch friendly, whereas the castrol 20w50 only has to be slippery. The BR and S lubes are roughly comparable in lube weight to 20w40 in their 80w versions and 20w50 in their 85w versions. They are not engine oils. Vintage parts are rare, getting rarer and not any cheaper, so why take chances over a couple of dollars.