Hi Tony,
Couple of questions to help narrow down what information might be of help.
First where is "Ganmain” and what is your normal out door temperature range?
Are you talking about your F60L?
If google is correct Ganmain must be in New South Wales, and if what I read on
http://www.coolamon.nsw.gov.au/files...inSnapshot.pdf
is correct 21.9-6.3C (71 to 43 F) the chamber of commerce information you have a lovely climate. It also means that you can probably run any mid temperature range modern oil. With leakage and seal life being more of an issue if the gear lub is to thin.
Over the last 30 years driving my HUP have gone from straight 90 Weight Hypoid to running the SAE 85W-140 which seams to be fine operating in any of my three trucks with out side temperatures up to 35 C – 95 F gear box, transfer case and differentials never have gotten too hot to the touch highest I’ve ever seen is 150 F – 65 C with an infrared thermometer and that was after more than hour at full speed. The multi weight lub has seemed better in cold weather. Leakage rates are more of an issue at higher temperature, new seals and speedy sleeves seem to be the answer to that problem.
Though you don’t get temperatures like this in your area at lower temperatures below 10 F -12 C the straight 90 along with the grease in the front drive shaft steering balls make it so that it requires 4 wheel drive to move the trucks on smooth snow. Without the front end engaged the wheels don’t turn they slide. The straight 90 also makes it very hard to shift gears or to shift the transfer case.