The 1988 GM service manual specifies "Syncromesh Tranmission Fluid" GM part number #12345349 for both the Getrag and Isuzu tranmissions. It is available from any GM parts shop and from most general automotive parts stores.
The GM specification No. is 9985648. It is a "Gear Lube - Special" by API standards
Your safe bet is to use a lube that conforms to GM specification No. 9985648. Multiple suppliers make transmissions lubes to this spec: QUAKER STATE®SYNCHROMESH FLUID
PENNZOIL® SYNCHROMESH FLUID
AMSOIL Synthetic Manual Synchromesh Transmission Fluid Others transmission lubes may also work just fine. For your application they may be better or worse than a spec 9985648 lube. If you have non standard applications, such as heavy towing or high RPMs you may want to examine:
Red Line D4 ATF; Red Line MTL
Valvoline Durablend ATF
Amsoil ATF
Mobil 1 ATF
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