pretty sure forum member Tornado has real world experience using RP for the gearbox. I have nothing to go by, since my 74 Mk2 is yet to start, and I have no hands on experience with anything related to this bike. i'm doing a full maintenance and service during the cleaning, detailing, and re-conditioning effort, and chose royal purple 85W-140 for the gearbox lube. there may be something out there that's better, but I figure RP can't hurt either. I've put enough thought into the matter, and time will tell....