"never lubricate servo motors or gears"

I'd have to say that as general advice that's sort of true, but very small amounts of a proper lubricant in the right places can do wonders.

Plastic gears may not "like" lubricating oils or light grease. The plastic may become brittle. There are "dry" "food grade" lubricants available.
Unsealed potentiometers may fail if contaminated by dry or wet lubricants.
Oils can thicken with age and even get "gummy". Oils can also attract dust and grit, causing wear out much sooner than expected.

Brushed motors don't generally "like" lubricants on the brushes, although there are specialized products made for
model racing car and electric train motor brushes. The products tend to allow the brushes to seat properly, and remove brush residue from the commutator.