Hi, I have an 11 years old Desktop 2418 with linear rails and I am concerned about proper linear block lubrication.

I try to keep the rails free of debris as much as possible but never lubricated them. They seem to be equipped with some kind of round lubrication fitting - they are not the usual zirks.

I'd like to know:
1. how these type of fittings are called ?
2. what type of grease gun adapter I need for those ?
3. what type of grease I should use ?

Thanks in advance.
P.s. I tried to do a search on the forum for that topic but the result mostly shows conversation about rack and pinion or cutting bit lubrication.