I’ve lost track of the specific synthetic product made for transmissions with brass syncros.

I use BG syncroshift. It is synthetic. Labeled for GL 4 use. But gear oil labeled GL 4  is used with brass syncronizers. 

Located in Harford County Maryland 

Most modern gear oils are safe with yellow metals.  Consult the manufacturers product info, if you are not sure.


Lawrenceville, GA

I went back through all the posts here and found my orginal post 2 years ago. At that time I found it on the Valvoline website. VPS Full Synthetic Gear Oil to replace GL4. It's not on their website today. I have to assume that they no no longer make it. My local parts store had it on the shelf back then. Its not there now. As I recall I had trouble finding GL4 when I put the car back together several years ago. Had to order it on Amazon or E Bay.

Did a little more digging. David Keys actually recomends not using synthetic in the toploader. Says it requires a certain amount of friction to work properly. As I recall the syncro rings are actually called friction rings. Going throuth my inventory of supplies I found the bottle of non synthetic GL4 that I used when I put the car together several years ago. I'm going to stay with it. Going with Synthetic GL5 Ford oil  and Ford Friction Modifer in the rear end. Its a 3.89 Traction Loc. Got about 500 miles on it and a little more on the toploader. Should be broken in and time to change all the fluids before I quit driving it for the winter.