Manual transmission fluid for 1987 Chevy v20 6.2L Diesel with 4WD

Have a 1987 Chevy v20 with the 6.2L Diesel and 4WD and manual 4-speed transmission. Recently had the transmission fluid flushed and re-filled. Ever since it has been shifting very roughly. Mechanic used 75W-90 Full Synthetic. Indicated it would take awhile to break in but curious if that’s the right fluid for this transmission? Also, noticed that the fluid in the hydraulic clutch reservoir is quite brown so thinking I need to flush that and put new DOT3 in as well.

Anyone know what is best transmission fluid to use on this truck? I seem to find everything from AT fluid like Dexron II or III but also have seen full synthetic 75-90 suggested.

Welcome any insight.

Thanks

Your truck has the SM-465 manual 4-speed transmission.

The recommended lube is 85W-90 GL-4 rated lube, ~ 4 quarts per drain/fill if I remember correctly. If you live where it gets really cold, most use synthetic lube. If cold isn't a problem, I'd stick with what came in it from the factory - just a high quality non synthetic, mostly for oil seal compatibility.

Welcome to the board!

same as your engine uses

I agree with Jim...85w-90 GL4 gear oil.........
The synthetic is super slippery and is likely the cause of the shifting issue.

The synchro rings are brass and are probably not liking the synthetic oil...

The synchro rings need to be able to grab the cone on the gear they are mated with and the friction between the two parts causes then to match speed....

I would get the box warm (Drive the rig 10 miles or so) Dump the synth and let it drain really good......Refill with the regular Dyno 85W-90 Gear oil.....

My bet is your shifty issue will go away.....

Synthetic oils are not always a good thing......Frictional quality is at times not the same and then parts that are supposed to drag a little (Synchros) do not do their job....

Also it is possible that the tranny internals are getting tired....And the super slippery oil was enough to bring things to light.....Try the regular oil....