This is essentially SuperLube.
MTN states:
Which is the same as what SuperLube advertises: Super Lube Silicone Lubricating Grease with Syncolon (PTFE)
No, they are different. I’ve used the MTN silicone grease a lot and like it, and I’ve used the general SuperLube for over 20 years now. Not the same things at all. In the most recent grease thread I added a post at the end where I purchased some of the SuperLube silicone grease (marketed as o-ring grease) and it’s really nice, darn near the same thing as the MTN grease. Who knows what Richard was packaging, good stuff, though.
OP, I think the Xtar grease you have is probably just fine. Apply it sparingly to threads…too much will just make a mess but also shouldn’t really interfere with conductivity (most gets squished away from the pressure and shear force as the threads are tightened and mate together so there will always be enough metal-to-metal contact even in designs where tube shoulders are butting together before the threads can really be mashed tight). The grey color you’re seeing right away is just residue/oxidation/metal dust that was left over after you cleaned…if you wipe away the new grease really well and reapply, that grey should be less and it probably won’t get as “dirty” over time, just depends on the host aluminum and quality of machining really.
I find it hard to believe that the grease MTN sells is not SuperLube since they use almost identical descriptions. Maybe MTN sells the o-ring version, but that’s not compatible with silicone o-rings. Or perhaps MTN sells another version.
But I guess I/we can’t be sure one way or the other. Glad you like the MTN grease you got though! 
Richard would have to tell us what he buys. SuperLube as a company does not actually manufacture every item they sell, either, so it’s always possible that someone found a buyable-source further upstream. Anyway, they’re all fine (save for potential compatibilities as you mentioned). I hope he’s doing fine and the business is sustaining…kinda wondered from time to time.
It appears Mountain Electronics is still doing fine, but who knows for sure. I placed an order with them last month. Seems strange that they still don’t sell the incredible 519As yet! I sure would miss them if they went away.
Thanks for the reply. The metal to metal contact makes sense. The host I applied it to was indeed a cheaper flashlight with low quality anodizing. Ill wipe the grease off and try with a new layer.