OPEN (EXPOSED) GEAR TYPE TUNING MACHINES:
Lubricate every other string change. If you change your strings every 2 weeks or sooner, do it once a month. Apply a very small drop of 3-in-1 or similar oil to each end of the worm gear and a couple of drops where the 2 gears meet.
CLOSED (SEALED) TYPE:
Need no lubrication. It's sealed inside.
If there is an adjustment screw on the tuning knob, don't over-tighten it. It will cause the worm gear to strip. The knob should turn freely with a small amount of drag.
If you see small metal flakes coming off the gears, it's time to have a new set of machines installed.
Gman ( o )==#