What fluid for a 4-speed trans?

I've looked over the spec sheet for the Pennzoil Synchromesh, and I think it could probably be labeled as a SAE 80 GL-4. Putting that on the label, however, might cause people to use it in an application other than it's intended use of a passenger car and light truck manual transmission fluid.

GL-5 has a higher level of sulfur and phosphorus compounds, which can be corrosive to yellow metals like brass synchros. So unless actually spec'd for a certain trans, GL-5 is best avoided.

I used to sell a good deal of the Synchromesh, and before that lots of Pennzoil GL-4 intended for manual transmissions. Funny how the GL-4 oil was discontinued shortly after Synchromesh came out. It was never called a direct replacement, but...