Oil/lube for Shimano freehubs
If you don't want to use a bicycle grease (Park Tool PolyLube 1000 or Phil Wood Waterproof grease or Shimano's greases or whatever), use something like automotive multipurpose or wheel bearing grease. 

The Valvoline stuff is red and comes in a big tub for about 5-10 dollars and can be bought at any automotive shop.