What fluid is in a 84 chevy truck 4 speed Manual transmission

8 pints For Manual Transmission: 4-SPD GL-5 All TEMPS 80W-90,80W
Notes: In Canada use 80W. Capacity: 4-SPD 8 pints

Automatic Transmission Fluid Capacities Automatic Transmission 3-Speed Pan Removal and Replacement 4.0 quarts After Complete Overhaul 6.9 quarts When draining or replacing torque converter, more fluid may be needed. Manual Transmission Fluid Capacities Manual Transmission 4-Speed Automatic Transmission Not Owner Serviceable Complete Drain and Refill 2.0 quarts

It it a 2X4 or 4X4? Is it a 2.5L 4-cyl, a 2.8L 6-cyl., or a 4.3L 6-cyl?
It matters.

4 & 5 speed 85mm transmissions use gm std. transmissin fluid

Just about all you can do is, with the engine off, from under the hood open the throttle all the way and check if the TV cable is tight. And check the fluid level, smell if its burnt or look if its brown.

Other causes could be,
Sticky governor
Sticking shift valve
Worn bands

Many years ago my mom's first 4 speed automatic was a 84 S10 2.8 2wd. She did not know it was a 4 speed and the indicator was broke so she counted the clicks to find drive. She was driving in the hills in over drive and all the downshifting was too much for it (should have driven in hills in 3rd gear). After that the transmission would not shift out of second gear.

Dexron III

You would need to find something that is the same as sychromesh fluid that GM uses.

9.5 quarts
The check hole is located near the oil drain plug.

You can use Dexron II or Dexron III, but you can also use the improved Dexron VI which replaces both the afore mentioned fluids. Use Dexron VI as it is an improvment. Caution: Do not overfill. Check fluid level according to the instructions in your owner's manual and make sure the car is parked on a level surface or you will get a false reading.

auto
3 speed 7 quarts
4 speed 9.5 quarts
manual
2000 or older 2.0 quarts
2001 or newer 1.8 quarts