Transmission Fluid

I have a 84 Chevy k1o 4x4 with a 74 350 and 4 speed manual transmission(serial# starts with a C). I want to replace the fluid in the trany but manual transmission is hard to come by and expensive. Someone told me to use brake fluid another guy told me to use automatic transmission fluid. I was wondering what the best replacement is. I had my drive train pulled apart twice now... once for a clutch fork and bearing and again for a new clulch. All in my back alley on the ground with a little extra muscle from a buddy. I dont want to tear it down again because my trany is fried from something as small as fluid. Any help would be a great. Thanks  

I took this from Novak Conversions web site.

When filling your 465 with gear oil, we recommend that you select a conventional mineral oil or a para-synthetic in lieu of a full synthetic oil. Properly assembled manual gearboxes do not have the thermal strains seen by combustion engines or hypoid gears. Synthetic fluid in these gearboxes, while not harmful, is probably an economic waste.

Hypoid gear oil is sulphurized higher than transmission oil and can be mildly corrosive to the non-ferrous alloys used for synchros, bushings and thrust washers in these transmissions.

An 80W-90, API-GL5 or MT-1 rated fluid is very good. Some claim faster shifts from using a 50W engine oil in their transmission and we do not consider this to be contraindicated unless you operate your vehicle in a very warm environment.

I have been running Mobilube SHC 75W90 in my son's truck for 4 years without any issues.  

I tried a  60 wt in my SM465 trying to get it to shift quick. It would not gear down no matter how hard I worked at matching the RPM. I quickly went back to 85w90 and raced it in mud pits and drove on the road with no more problems.I would recommend sticking with a good quality gear oil. I think I was using Valvoline.