There are two reasons for the grease. Aside from lubrication, some different metals will bond to each other, e.g. alu seatpost in a steel frame. In the case of Titanium, you have to use a special grease laced with copper powder.
But otherwise, any grease is fine. I use
Phil Waterproof Grease for most things, but also have a little stick of white, silicone-based, grease that I use for some things.