The suggestions by all the above would be fine by me also. I happen to normally use what Ted suggests. I use Mobil 1 syn grease because I have a greasegun full of it for my truck. But I'm of the opinion that just about any kind of lube will do the trick as well as anything else in most cases. Temp range of the lube may be important to those that will use their guns in extremes. I'm not sure how the Vaseline mix would do for 120F dove hunts or late season phez in the arctic (north of Bakersfield, to me). The specialty gun lubes seem very expensive and I haven't seen anything to suggest they work better than Mobil 1 auto grease.
