I know I will incur the wrath of purists everywhere...but I just stab the can with a closed pair of fine nosed pliers kept in the top of the aircraft toolbox(obviously ensure they're clean first). Makes a neat round hole and because the pliers are of tougher material than a "church key" they won't produce metal debris.
If making another vent hole on the opposite side of the can by this method, just make sure you cover the first hole, or you risk wearing the oil yourself..
