Lubing CO2 pistol slide

People say to only use silicone based products, oil on the tip of the CO2 cartridge, and the manual says to use silicone oil also to clean the barrel.

I used silicon grease for the slide rails since I've read silicone oil is not a good metal lubricant.

Anyway, the slide feels quite gummy and sticky, it seems to work nicely since I'm at 3000 rounds approximately, but I'm not sure this is the best solution.

Should I keep using silicone grease, or I'd rather using something like Ballistol ? The grease residues are a bit black so I'm not sure it's fully preventing metal wear...

Pistol is an Umarex SA10