I need to replace an axle shaft but for the mean time I believe that I have lost almost all my transmission fluid. My research gave me a GM part number for the fluide I need though I was wondering if anyone knew of an off the shelf brand I could use on my 1990 3.1 5 speed transmission. Also to refill the trans must I go through the speed sensor location? Just a few Qs before getting into it....

The only fluid to use is Synchromesh. There is no other recommended fluid (yes, I know people claim using 5w30 is ok, but its not). There is a fill hole with a dipstick on the trans, you don't need to use the VSS hole.

5w30 is what is used in the older muncie 4 speeds. i did use it with an additive in my getrag 5 speed, but it didnt work as well as syncromesh.

some parts stores carry penzoil syncro oil or gastrol syncromesh. some even carry the gm syncromesh. any of those work fine.

Autozone carries Syncromesh....2 quarts cost me like $6...