Gearbox oil!

Want to change the gearbox oil so was wondering what oil I should get been looking at redline oil on Opie oils but not sure the grade seen a few posts on here 75w90 is that the correct grade? Has anyone got any recommendations would be great!
 
Dead easy for the garage. Get oil warm, onto hoist or pit; undo g/box oil plug in bottom of casting, oil into waste holder. Refill plug is a finger tight, threaded plastic cap plug on mid-upper part of box casting on side of the box (rh from front viewed backwards) use left hand from behind/ below side to remove. Replace bottom plug and use oil hypodermic to squeeze the new oil in. Replace plug. Job done. Well worth it!