It won't hurt if you add too much. It could drip, but nothing really to hurt. And it would be more likely to spill when you move the organ if you put too much in.
The idea is to keep the sponges which hold the oil damp, not dripping wet.
I forget what kind of vibratto the L's have, I am familiar with the M3 scanners. These have a small cup, so be careful when oiling the scanner. On the M3 you can open the cover and actually see the cup and touch the sponge. Your fingertip should be wet after you touch it. If it drips you put too much in, no big deal, you can probably go a bit longer without oiling.
The TWG is basically the same except the trough for the oil is much bigger and harder to get to.
If you intend to move this organ around a lot, ie gigging with it, then you are better off putting less oil in more freqently.