Chevy manual transmission. WHAT OIL?

Just aquired a 93 and a 95 Chevy pick up. Both are 6 cylinder, standard trans (5 speed, no creeper gear). I want to change the oil and the owners manual say syncromesh oil GM part 12345349. That is $16 a quart at the dealer. Should I get that or what is a good compatable oil for this application. Back in the day they were all 80-90 gear oil. I know my 95 Ford uses ATF. Any help would be great.

Use the syncromesh-it’s good stuff. Any other substitutes I might suggest will be equally or almost as expensive.