So I got a 1990 22re pickup 2wd with a 4speed manual transmission I have 8090 oil in the transmission with 40,000 miles on the oil
so the question is would u change it ?
if not how long would u keep the oil in there?

 

Based purely on mileage, that seems like an alright time to change it. Examine the color of the oil and look for any junk in it and decide if you can go a little longer on the next change or if you should maybe change it sooner.

I aim for around 30,000 miles on the transmission oil but 4x4 trucks with big tires make the transmission oil's life harder.

You didn't ask, but I use Redline MT90. Many people here seem to also like that stuff.

 

There's no specification in the FSM on replacing the transmission oil. Just inspecting it.

Me, I let mine go in my 2WD for about 50,000. When I go to change it, it's looks virtually new, unused. In the 4Runner, I replace it every 30,000. I started that schedule when I lived in Yuma, Az, which is a very hot place to live.
Now that I live up here in Oregon, don't get me started on our pols, I use the pickup in the summer, the 4Runner in the winter. I change the 4Runner every other year, now. Why? I drive in snow a lot now. Even so, the oil I remove still looks new. Feels just as slick, to my fingers, as well. Whatever that's worth. If I grind the gears by accident, of course, I get some metal shavings on the magnet in the drain plug, but that's the worst of it.
Heck, I took my truck in to be looked at a couple years ago. they put it up on the lift, and the mech checked the oil in the transmission, transfer case, and 2 pumpkins. He asked me if I had just changed it. I told him no, it's been a few years. Amazement

Finally, that MT90 seems to be the best I've ever used. Makes the shifting easy, and the driving smooth. In 4WD or not. It's just darn good stuff. I don't have huge, knobbly, mud running tires, though. Just the good old BFG AT-TA Radials. 31". They give me good grip in the snow. I'd like to get a set of studded ones, but my lovely wife says no, so what are ya gonna do? I just have to be VERY careful on icey conditions.

Good luck to ya!
Pat☺

 

FSM usually didn't get into service intervals, they let the owner's manual do that most of the time

edit:bitd there was a bit of a game being played amongst the manufacturers regarding the wording. Consumer reports would publish the cost of maintenance over the first three years based on the service schedule in the owner's manual. "Inspect" had a zero value, which kept the cost of maintenance low. But if you read the description it said "Inspect, and service or replace as needed." Fine for wiper blades, not so great on a powertrain oil.

 

Agreed on Redline MT-90 for the transmission, which is GL-4. Redline 75W90 GL-5 for the transfer case and differentials.. This assumes MT5 transmission and no limited-slip differentials.

Replacement interval is a combination of time and mileage. Time-based to address moisture accumulation and age-related breakdown, and mileage-based to cycle out contaminants. I think some trucks have oil from the factory and it's never changed and they do fine.

 

I used a "house brand" gear oil when i first changed out all the fluids. I eventually switched it out with redline(diffs/trans/transfer) and it made a noticeable difference in reducing sound.