Manual transmission fluid?

[6872xtc]

Maybe this would be sufficient

Mobil Delvac 1⢠Transmission Fluid 50 | Mobil Delvac⢠Transmission Fluids

Learn how Mobil Delvac 1™ Transmission Fluid 50, a full synthetic, manual transmission fluid, was designed to provide maximum engine protection.

www.mobil.com

Add a pint of Lucas HD oil additive if desired

I wouldn't worry about adding anything to that oil. With all of the heavy duty specifications that it meets, it has to be miles better than the 1978 80w90 in a light duty transmission.

Or try what @Bent77 suggested.

 

[CT-BLAZER]

Redline makes multiple Manual Transmission Fluids in different viscosities....

Harsh shifting is probably more of a wear issue.

I've ran Slick 50 gear oil in my dually with no issues ....

Mobil 1 makes two or three viscosities for manual transmissions....ran them in rear differentials with zero issues.

 

[Chevy305]

I definitely read some literature somewhere that said it was okay to run straight 50W gear oil in cold climates. I tried it one winter years ago and it did make a difference in cold shifting.