Morris Minor Owners Club

OK. We all know to put engine oil into our gearboxes. We all know that as far as the engine is concerned, the SAE rating (SG, SF etc) is as important, possibly more so than the viscosity (20W50 etc), and that it's worth spending a few quid more for a better SAE rating.
How important is a better SAE rating for your gearbox? Is it worth the extra few quid, given that the gearbox oil doesn't get as hot or come into contact with acids, combustion by-products and other general nastiness?

It's very small capacity - don't worry about - just use the same oil as you are putting in the engine - although I would suggest 10W40 for gearbox - unless it's on it's last legs in which case 20W 50 !