On my WM18 I just squirt some aerosol white lithium grease through the small hole on to both the spindle gears and intermediate gears while the mill is running slowly. It doesn’t need doing very often. If you ever tilt your mill’s head there may be access to the gears from the back via a thin access plate, which will let you get a better look at what you’re dealing with.
There is no metal-on-metal of the gears on my mill; metal gears couple with plastic.
Putting any significant quantity of oil in the gear case of my mill wouldn’t work too well as the oil would seep out of the rear access port very readily and you would always be left with only about 1/2” of oil at the bottom of the gear case, which would do nothing except prevent rust forming on the gear case’s floor.
As for the quill, I put a thin coat of Mobil Vactra no. 2 or 4 on it. This needs doing even less often than the gears.