Tatume,

With my cheap but very accurate Knight inline, I used a water based industrial grease until a very experienced friend told me about the long term problems with what I was doing, and I stopped. The rifle still shoots nicely so I must have stopped before any long term damage was done. That said, a very good friend of mine who shoots vastly more than I do, uses grease in all of his muzzleloaders - both traditional as well as inlines and does not seem to have any problems.

Good hunting!