[quote=“Dog’s_Breakfast”]Hi all,
It had just occurred to me the other day that I’ve now owned a Mitsubishi Freeca van and a Toyota Zace truck for approximately 12 years (each) which I bought new, and I’ve never changed the manual transmission oil, nor the rear axle oil. Nor have I checked the levels to see if any oil needs to be added.
Well, my bad. I know that you must change automatic transmission oil after so many kilometers, because it starts burning and turning brown (it’s deep red when new). But I’m not really sure about 90-weight manual transmission/rear axle gear oil. Does it actually need to be changed, or just topped up? I would like to know - quite a difference in price between topping up (maybe NT$100) versus changing (NT$1500), and I need to double those figures since I have two vehicles.
cheers,
DB[/quote]
Manufacturers recommendations probably vary but IIRC none of the RWD vehicles I’ve had came with any recommended change interval for the rear axle or gearbox, and (also IIRC) there was no recommended change interval for the automatic gearbox in my Sierra (I should be able to check that since I’ve still got the Haynes somewhere, just dunno where)
You’d expect the anti-scuff stuff in the hypoid oil to get used up but maybe it just goes back into solution/suspension and gets recycled. You’d also expect the oil to shear down, but maybe its way over requirement to start with.
I did change the gearbox oil on my Marina (yeh, yeh, very funny) because I drained it (can’t remember why, maybe the box was noisy) and there seemed to be an awful lot of metal in it, plus a couple of needle rollers. That was a waste of money though, cos it was clearly already clapped out . I was going to try adding a nylon shirt (an official Arthur Daley scam handbook trick) if it got worse but it never did. You can do stuff like that with a clapped out Marina. 
For gearboxes, at least in some cases, I think hypoid oil may not be too critical, since (again IIRC) some manufacturers (eg SAAB) changed their recommendation from hypoid to multigrade motor oil. For peace of mind, though, you should probably stick with the recs.
I’d just top it up. If I was concerned, AND I was confident I could drain it without contamination (difficult but possible if you give the area a good clean first) I’d drain it and have a look at it after settling in a pop bottle over a magnet. That’ll give you some indication of wear and then you can decide whether to put it back in or not.
Depends somewhat on your plans for the vehicles.
But dont listen to me. I just gratuitously wrecked my RZR gearbox. 