Hi I did a search and came up with nothing so I'm gonna post this.
I know this is kind of a stupid question but I'm a little confused. A few years ago I had my t-5 seize up on me and my friend told me that some dumb @#$ put automatic transmission fluid in it. So when I installed a new tranny He told me to put gear oil in it. Well I have to change the fluid again and looked up in the shop manual what fluid is supposed to be in there it it said auotmatic fluid. So which one should I use? Do you think I could have damaged anything while using the gear oil cause it is so thick or is it better then the auto fluid?

 

Either will work fine in your transmission. Whatever it was that actually happened to your old one, having ATF in it wasn't what killed it.

The factory waffled back and forth a couple of times in their "recommendations" for those old 1st design T-5s. Basically, they wanted to use ATF for the improved gas mileage; and gear lube, for improved durability. They changed the fluid specs, even when the parts themselves, were absolutely identical in every way, part # for part #. There was no clear-cut "winner".

The later ones, with the composition synchro clutches, should have ATF.

Being as how you have a 6-cyl, I don't think transmission durability is much of an issue. You'd benefit from the lower drivetrain losses from thinner lube, so you can get every last gerbilpower you can to the wheels. I'd suggest ATF.

 

In 1976 I bought my first new car (a Pontiac) with a borg warner 5 speed. It used dexron II in the transmission. I eventually rebuilt it at around 90,000 miles but it was because the rubber boot on top of the transmission (where the shifter went into the top of the transmission) had failed and the transmission was full of road dirt/grit. I can remember guys a few years older than me talking about using automatic transmission fluid in their muncies and t-10's because they said it seemed to make them "shift slicker".

By the way. My 1/2 4wd PU also uses Dexron (III I think it is by now) in the manual 5 speed and in the transfer case.

 

I'm with RB on this one. Use Dex III. I tried to run 80/90 gear lube in mine, and although it seemed to work OK, I had a problem with grinding while changing gears when everything is cold when first started in the morning, and it was a pain till the trans warmed up a little. - But it works fine after warm.

Oh, and don't cheap out too much on the Dex III - 3 different tranny shops recommended Mobil-1 synthetic Dex III exclusively. Not sure why - maybe it lubes a little better than the non-syn??