After getting a nasty burn from picking up a Williams loco and touching a hot roller, I always lube rollers. The postwar Lionel instructions to not oil rollers are not appropriate for modern trains where cheaper metals are used in the roller. The small diameter of the roller means the rotation is faster than the wheels and friction happens. I use my normal oil - sewing machine oil - in a hypodermic needle applicator to put a minimal drop in the roller axle. Works well for locomotives and lighted cars.
David Farquhar describes why a minimal amount of oil improves conductivity here. Corrosion is the enemy of conductivity. Conductive oil or grease has an additive (graphite, or a metal compound) that improves conduction, but I haven't seen a need to use a specialty oil yet.
