There's a special lubricant for sunroofs that I get from my local M-B dealer -- it's their special part no. for that exclusive purpose. I don't think M-B sunroofs and Volvo sunroofs are all that different, and I've found that in applications like that, M-B's parts are "overkill" quality, but it's not that expensive that we wouldn't want the best stuff for our old Volvos, either, right?

Anyway, the p/n is A-001-989-46-51, and it's called lubricant paste -- it comes in a 500g tub ("tub", not "tube", btw) that includes an applicator (brush), and contains enough for about two lifetimes (about 50 cars)! You'll never need to buy it again, ha, ha.

And it's much less viscous that white grease, which IMHO tends to dry and stiffen with time -- this M-B stuff is more like vaseline than white lithium grease. Personally, I use it for other things as well, such as the hood lock mechanism, etc.

Good luck.