Have now changed all my transmission oils to fully synthetic & can definitely tell the difference. Have changed as follows :-
Diff = Fuchs TITAN SINTOPOID SAE 75W-90 synthetic gear oil (from Opie oils)
IRD = Fuchs Titan Race SYN 5 75W-90 Fully Synthetic Gear Oil (from Opie oils)
Gearbox = Difflock Evolution 1 Fully Synthetic Gear Oil (from OEC International)
Changing the diff oil was awkward as there is no drain on there. You can see where LR planned to put one as the differential casting has the boss on it; but it is just not drilled out for a drain. Getting the oil out was either by removing the end plate or syphoning out the oil. I did the later as the end plate is very thin (I took it for a run to warm the oil first). I purchased a 100ml syringe with length of hose (about 8mm diameter) off e-bay for £2 and dragged all the oil out. Then put the new oil in using the same syringe. 40 minutes job done.
The gearbox & IRD I did at the same time - (1.25 hours total time).Took the sump guard off first, then went for a spin to warm the oil.
Draining the gearbox was easy. Allen screw filler and drain plug (8mm). Made sure the filler came off first then dropped all the oil out the drain in to an old washing up bowl.
Whilst the gearbox was draining out, did the same to the IRD - socket straight onto the drain plug. Again, made sure filler came off first (socket on a long extension) before touching the drain and then drained off into another old washing up bowl.
The oil which came out of the gearbox looked ok and in good nick (77,000 miles on clock - think this is the original oil) whilst the IRD was black and a bit sludgy on the magnetic plug.
The gearbox has bonded seals on the plugs which cleaned nicely with some WD-40 whilst the IRD has copper washers. None required replacing.
When replacing the IRD drain plug TAKE GREAT CARE. There are only a few threads on the aluminium casting of the IRD and when tighten the plug up, it felt as though I could easily strip this thread.
Filling the IRD was dead easy as the Fuchs bottle fitted perfectly & I did not need the syringe.
Filling the gearbox definitely needed the syringe. However, the Difflock oil viscosity is such that it flows out of the syringe really easy.
All in all I am pleased that everything seems to run smoother now. Was it worth the extra cost for synthetic oil? Well that remains to be seen. I have not yet used a full tank of diesel to see if there is any economy improvement. I will leave it for a while and let you all know if there is.
Big journey to top of Scotland soon, so will know more after that.