Recently bought a 91 4cyl, don’t think transfer+ tranny + diff fluids have ever been changed. Should I leave them be? If not what fluids should I use?

 

It is your Vehicle it is your call if you change fluids or not.
only you know your plans .

Every Toyota that crossed my path gets all the fluids changed

First always remove the fill plug first!!!!

It is not good to drain things then be unable to get the fill plug out

I can tell you the recommended gear oil some swear by fancy after market products API-GL5 or better 75W90 transmission and transfer case and differentials

Myself I used 85W140 in everything because I had a life time supply.

 

I've read often that the tranny and differentials take different AI-GL- ratings. My brain being the sieve it is, I can't recall for certain which gets the GL-4, and which the GL-5. Having said that, I think that the tranny get's the GL-5, and the differentials the GL-4, but I may well be totally bass-ackwards on that. Do a search for "gear oil" or GL5 or whatever in this forum. I know it's in here someplace.

I personally, used the 85W140 when I was in Yuma. That place can get a bit warm, at times. Like 110° F in winter. The heavier oil is great for high temperatures like that. Now that I live in Oregon, I've gone back to the 70W90 weight oil. It seldom gets much above 100 here, and that's a rare day. More often, in winter, the temperature is down below freezing. The lighter weight oil is perfect for those conditions.

I change my gear oils every few years, or at 60,000 miles, whichever. Last time my mechanic checked the tranny oil, a couple years back, he said I had just changed it. I told him it was 5 years old.
Be certain you can get the fill plug out before you go to draining anything, as was mentioned above. Sucks to not be able to get the fill plug out, but you've already drained it, and need to drive it to a shop to get the fill plug out. The first time I changed mine, on both the 87's, I had to put a large combination wrench on the fill plug, and tap it firmly with the rounded end of a small ball-peen hammer. Worked great.

You can pull the fill plug out, and stick a finger in to get a sample of the oil, to see if it needs to be changed. It may NOT, even after 100,000 miles.

Good luck!
Pat☺