THANKS Tim
i just used the same autozone to look up S10 1983-1991 S10 2 WD
AND IT SAYS:
Transmission
ALL GM Blazer/Jimmy/Typhoon/Bravada 1983-1993 Repair Guide
Home
Transmission Fluid Type
For your 1991 Chevrolet Truck :
SPEC-FLUID-TYPE/WT
***IMPORTANT*** THIS MANUAL TRANSMISSION MAY CONTAIN DEXRON (USUALLY ASSOCIATED WITH AUTOMATIC TRANSMISSIONS) OR GEAR LUBE. **USE TYPE AF (ALL CLIMATE) THESE ARE ALL OEM OPTIONAL FLUIDS DESIGNED TO SATISFY A VARIETY OF TEMPERATURES, CONDITIONS AND MANUFACTURER REQUIREMENTS. DEXRON USUALLY HAS A THIN-REDDISH CONSISTENCY AND COLOR. GEAR LUBE IS MUCH THICKER IN CONSISTENCY AND HAS A PUNGENT ODOR. DETERMINE WHICH LUBRICANT IS REQUIRED BY CONSULTING THE OWNERS MANUAL, VISUALLY VERIFY BEFORE PROCEEDING, AND REFILL/TOP OFF WITH THE SAME TYPE OF LUBRICANT.
http://www.autozone.com/servlet/UiBroker/?UseCase=C001&UserAction=answerRequiredQuestion&Parameters=%7C%7EWITH+MANUAL+TRANSMISSION&PageSync=8