It doesn’t matter whether it’s silver brass, copper, nickel…What matters is whether the slide itself was made as a dry fit or a greased fit. The dry fit which is used particularly on the modern Boehm flutes but also by some makers on the simple system flutes is made to a very close tolerance and grease isn’t needed in this case - in fact greasing the tighter, dry fit can even cause these slides to jam.
The dry fit has to be used for the modern Boehm flute since they typically don’t have a barrel so the flute has to be taken apart at the slide when the player is putting the flute away and if there’s grease on the slide, it will then pick up grit and dust causing scratches in the slide when it is put together again - and that’s independent of whether the metal is silver or not.
Most makers of the simple system flutes used in Irish music make the slides as a looser, greased fit and it’s the way I make mine. They can be done this way because mostly they are made with a barrel which remains together with the head section when the player puts the flute away so the greased section is not exposed to grit and dust. They are easier to make this way and there is little chance of a leak if they are kept properly greased.
So unless Hammy has now gone in the other direction and has moved to using a dry fit for his slides instead, I’d say try a thicker grease. It would also be better to email him before squeezing the inner tube as he may not see this thread. Others on the board who have one of his flutes might also comment.
Garry