Light machine-type oil are most common for glass cutter blades. Some companies will sell specialty glass cutting oil as well. Light oils are used on blades, not on the edge of your cut - which you definitely want to stay dry. It's important that you only apply the oil on the surface of your blade, or you can put too much pressure while cutting and it could wear out prematurely. But make sure to change your blade often if it becomes nicked or excessively marked. Especially if you're using an electric knife, clean it regularly because otherwise dust particles will damage the motor over time.