I believe the Lucas gear additive is the same for both transmissions and differentials. I have it in both but cannot tell you the mix ratio offhand. It should be specified in their brochure.
I have found it to be excellent, or my shifting is getting better!
As to water in the trans the only thing I can think would be a leaking cover gasket. However I am no expert and someone else may have a better idea.


1949 Chevy 1/2-Ton
"Sedgewick"
In the Gallery
1989 Caprice