Is this a classical/Spanish guitar? I'm supposing that you are talking about the capstan shafts in slots in the head. There's no mechanism in them.
You could mean that you have machine-heads, on a steel-strung guitar, and they may have plastic caps on the back of the head, or white grommets on the front of the head.

I don't know if there are plastics that react to alcohol, I've never heard of one. Can't advise you there. Some plastics react badly to Turps and other solvents.

All tuning mechinisms I've seen are easily removable, and fairly cheap to buy anyway. Cheap ones are cheap to buy, I should say.

I am still saying you shouldn't oil the belly. Much as I like an oil finish. On furniture.