Bloke i have worked with for years built a fwd sierra 4 door and stuck the lucas oils in the diffs and swore by it, he reckoned it even made the car easier to push!
I tried the diff oils in my discovery never noticed any difference but it always had syn oil anyway.
Landrovers always had manual gearbox troubles, good place for oils is the difflock site they have their own online shop, not cheap but i can vouch for the R380 gearbox oil very very good.
I did try some of their pas stop leak in a scania 4 series years ago to put of buying a pas box and it actually worked for a while anyway! not forgetting they were doing 2000 kms a week.
Lynall