Some more notes about cans of grease in Nethack:

The game actually allows you to grease any item, not just weapons and armour. Because grease protects against water damage, it's a good idea to grease your bag of holding, so that if you fall into water the contents will be unharmed.

If you are attempting a genocideless ascension, grease your helmet for some protection against mind flayers, but remember that a master mind flayer may wear off the grease and eat your brain in a single round.