Just be careful with friction additives. Remember many of them are designed for gearboxes that use brass synchro rings. Most of your modern boxes like the T5 and T56 use organic facings for the synchro rings much like the clutch and brake bands in an auto transmission. This is one of the reasons they specify Dextron fluid.
Here's a pic of a synchro ring from a world class T5
Just check with the manufactures recommendations as to whether the additive is suitable for modern gearboxes.
Cheers