How do greases work?

A grease is a semi-fluid product made up of a lubricating fluid, a thickening agent or soap and performance additives.

Thickening agents are divided into soaps, gels and mechanical thickeners.

How soaps work:

As far as soaps are concerned, the most common representation is of a sponge which contain the elements (oils, liquid and / or solid additives) that lubricate the contact (ball on a track, meshing of gear teeth, etc.).
In a roller bearing, for example, the rolling component, just like a steamroller, compresses the grease. This releases oil, preventing all metal-on-metal contact and facilitating the rolling. During the idle phase, the grease partly recovers its oil. For the same type of soap, a grease with more soap will have a tendency to release less of its oil. This phenomenon is more or less desired depending on the application (long service life), bearing in mind that if too little oil is released (the term used is "bleeding"), the contact will be improperly lubricated. It's a question of getting the balance right!
Gradually, the steamroller – like a sponge that is subjected to a scissors action – shears or breaks the soap. The grease then loses its consistency, is no longer able to retain the oil as effectively and so escapes from the roller bearing. The parts then have to be re-greased.