Oil made specifically for glass cutters. Anytime you cut glass you should be using cutter oil to avoid blocking the cutter wheel with little bits of glass and keep the cutter moving cleanly over the glass surface while laying down the score line. Better glass cutters include oil built into the cutter itself. You can use regular cutters that do not have cutter oil built-in, but it does take some practice and technique to keep the oil from getting all over everything.

Cutter oil is also designed to wash and wipe off cleanly without leaving a residue. Don't attempt to use any other type of oil to provide lubrication. A 4 oz. bottle runs about $5.