Manual Transmission Oil on 1990 GMC 1500 Pickup

Hello Team,
I recently purchased a used 1990 GMC 1/2ton pickup (SL Sierra) with a 5.0L engine and 2wheel drive and am seeking to determine the proper gear oil to use in the 5 speed manual transmission, what do you recommend and why?
Thanks,
Alaska GMC

Asked by for the 1990 GMC 1500 Pickup
The recommended gear oil for your 5spd gear box is something called - GLS; GM part no. 12345349. I have not been able to find an specific information on GLS anywhere. I would suggest to check with your local GM dealer and see what the recommend for your area. The requirements in Alaska may be a bit different.
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GM Performance 88900333 Synchromesh Transmission Fluid