That's really interesting, I had a look at the several empty gearbox oil containers I had and both were marked as GL5.
I also have a container of motul gearbox oil (for a guzzi I used to own) that contained molybenum for the load in the diff and that had both the notations GL4 and GL5.
In my plunger B series that runs straight 50 in the engine, I've always just run 30/40 over the last 25 years.
Apologies for hi jacking this thread that was about oil leaks!
Yep, but not the GL5 spec oils that were designed for the rear axle gears. These can cause problems, so be careful!