Sorry guys i must have fallen asleep until someone walked past and accidentally kicked the creeper ! Ok if you have a worn diff you need to go for the heaviest viscosity without risk of lube at the critical areas. What you can use is a straight SAE 140 also known as ISO 460 (no not 85w/140 but straight SAE 140) in GL5 performance (contact your favourite oil co. Castrol SA had an ISO460 which met gl5 pe rformance level. Unfortunately this is not a magic fix and will only reduce the noise level to a point. Failing this you will have to rebuild.
Synthetics - yes well now how deep are your pockets? Synthetics are great in the right application. Personally if you wish to go synthetic then fill your gearbox and axle first. This is a good investment. For the engine ? unless your engineering and mods are on the ragged "acceptable" edge, stick to mineral and drain more often. Dust and deposits do far more harm and they must be ousted.
Using ATF in manual transmissions where the manufactrurer okays it is fine. but just because it shifts nicely does not mean you can go against manufaCTURERS RECS. BEWARE.