I have searched several threads on here about the proper fluid to use in your A-833 OD. I am still somewhat confused. First off I live in North Carolina so we do not have a lot of cold weather. Second this vehicle is a daily driver occasionally pulling a small utility trailer with less than 1,000lbs if even that. What does the service manual call for in an 87 D-100 two wheel drive and what do most of you use that works best. I put in Amsoil Severe Duty SAE75/90 weight and I had to drain fluid to remove trans for another issue but the fluid was very dirty for only driving it less than 10 miles after engine and trans install. I have seen motor oil,ATF,and regular gear lube for suggested use. I am trying to find out what works best.

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!987 D-150 Short Bed,Slant-6, RWD,A-833,2007 Ram Mega Cab 3500 RWD Cummins 5.9

It's kind of up to the user and his preference:

The manual says Dexron Mercon ATF.
It works...But I've seen evidence is a couple of trannies where the ATF didn't stay where it should have if the vehicle stood unused for a while then the bearings and other surfaces were not protected well (I had a mainshaft the the nose was trashed on due to this.

Some old timers will use GL-4 90wt...(not so great in the winter, which sounds like you don't have this problem). Make sure it is GL-4 for Manual Transmissions...hypoid oil for rear ends and GL-5 will cause problems with the synchros properly doing their job.

Some people have had success using a standard 30wt non-detergent motor oil, similar consistency to 75 wt or so...non-detergent is needed as regular motor oil's detergents break down over time and become acidic and that will tear away at the synchros/etc.

Quite a few people have used Redline Manual Tranny lube...

Take a stab at which suits your average temp, how hard you drive it, and go from there...Most likely your oil blackened because the tranny hasn't been cleaned out in a long time and the new oil just washed out some of the nasty old varnish in the corners and burned up old oil from years gone
at the bottom of the case that didn't get 100% drained out.

-D.idiot

GL5 is too slippery for the synchros as DI says. Switching from 80W-90 GL5 in a good shifting A833 to Red Line MT-90 made it a great shifting box.

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Joshua

My Factory Service Manual and Owners Manual states that ATF is the factory fluid, BUT, that if you don't like the shifting quality to use 90W. I HATED the terrible shifting quality associated with ATF. Shifts were terribly difficult and double clutching was the norm. I drained the ATF and went to 90W...Mobil-One. Best low dollar improvement you can make...

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Slanted D150

Since a dirty fluid condition was mentioned in the op, I'm thinking running ATF for a short period would clean it up better than gear oil type stuff. Does that sound like a reasonable idea? It's cheap enough to try and switching to heavy stuff afterwards shouldn't be a problem right?

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1966 Coronet Deluxe

I have a main shaft from the original transmission in my feather duster, when I disassembled it the nose on the mainshaft was galled badly, the previous owner followed the factory recommendation of ATF and I notives that unlike oil that leaves a nice film on everything even after sitting a while, the ATF was thinner and tended to drain out of the places that needed to stay lubed...eventually the nose on the mainshaft without constant protection just didn't make it under the pressure of the roller bearings and input shaft when in 1st and second while driving in town.
If the tranny hasn't been apart in a while and the oil is dirty, it just makes sense to overhaul it, clean it out and see what's at the bottom of the case, and look at critical parts (mainshaft nose, bearing retainer clearance, countershaft bores in the case-can be egged).

2 cents...

-D.Idiot