Can I use Wynn's Engine Stop Leak to stop Gearbox Oil Leak

Hi guys. I've got an ex-RAF 90, 1991 C Reg 2.5 litre diesel, manual gearbox. It's got a persistent very slight leak of red oil from the gearbox housing. It doesn't warrant stripping it down for major work etc, but my brother who lives out in Canada and used to be a mech in the REME said to stick some Wynn's engine stop leak in it and that should plug up the drip.

It says on the tin -

Will have no effect on the viscosity or performance of engine oil
Is compatible with all mineral and synthetic, single- and multigrade oils

So next question is what should I be topping it up with and is it compatible with this stuff? Does it really make a difference the quality of the oil you top the manual gearbox up with? Some places rave about Difflock Evolution 1 Fully Synthetic Gear Oil. I don't want to drain it off and fill it from new.

Might help if we knew what gearbox it was (Tranny or main?) engine?

Also where is the leak coming from? If its just the inspection cover I would drain it, take it off, clean it all up and re-fit with a bead of RTV.

I wouldn't bother with evo stuff. The standard stuff (ATF or MTF94 depending on box) would be fine.

G