Sounds interesting and promising. I love stuff like this - nothing like garage trial and error with lubricants. I have not seen this gearbox, nor do I know the design - so I am not an "expert".
I know lubes and greases pretty well, so I will just say a few things/ask a few questions.
1) For wear protection the brand of "stop smoke" may matter. Some do have antiwear agents, some are just very heavy so called bright stock base oil with viscosity improvers (thickeners). In some gearboxes this VI may shear (thin) permanently.
2) Why didn't this gearbox come with fluid in the first place?
3) What type of grease is recommended?
4) Does this gearbox get hot with use?
5) What is the exact design of the clutch? Does grease normally get on it?
5) There are SAE 250 gear oils made, that have huge room temp viscosity, even at 100°C the viscosity is over 500 cSt. Then no need for the 50% thickener, which in my opinion could possibly lead to early gear wear - I'm not saying his with any certainty, but it's something to be considered. OTOH an SAE 250 gear lube may not be right at all for the clutch. Lubrication can be a series of compromises.
Lemme know - again - not trying to scare anyone. It's just my mind I can't control…..it just gets to racing when lubes are involved!