Non-Overdrive gearbox fluid question???

["Brad Eells" <bradlnss@lightspeed.net>]

Hello everyone,
I have a question about the proper lubricant for non-overdrive gearboxes:

I have for many years used the Valvoline synthetic racing gear oil (75w-90 I
believe). I recently read a Roadster Factory publication about the corrosive
effect GL5 spec fluid may have on brass or bronze parts. Their
recommendation was a GL4 spec fluid.

Any thoughts about GL5 vs GL4 vs synthetic? I have always felt synthetic was
the way to go, but now...

Thanks,

Brad
62 TR4