Technical GM manual transmission oil, GL3 or Sychromesh?

I’m going to check all of the fluid levels in the 3 speed and rear end of my 1955 Chevy. I have seen GL3 advertised but spendy and on back order, whereas Sychromesh is half the price and readily available. I’ve read where you cannot use GL5 in old transmissions as it affects brass. Has anyone here replaced GL3 with Sychromesh? (No, I’m not going to use trans oil in the rear end.)

 

GL-4 gear oils are fine.

Syncromesh is fine.

 

I’ve been using the Driven GL-4. Says safe for all white metals on the bottle. Not sure on availability.

 

I run a Richmond GL4 in my M20. One guy I know with a shop swears by Syncromesh. But the ones I have seen do not have GL4 on the label, go with what you know.

 

Tractor Supply carries a GL-1 for older Ford tractors and comes in 1 gallon containers for a reasonable price.

 

I always used Red-Line from Summit.