Oil exposed to hot gas bakes into a varnish-like substance.
Clean and wipe off only.
The gas cylinder attached to the barrel is fairly self cleaning
due to the high pressure of gas at that point (much like the
cylinder and short stroke piston on the M1 carbine, in normal
use, there is no build up).
The AK piston head of the operating rod should be cleaned and
wiped dry. Even the stainless ones will get stained black,
so don't get obsessive.
The AK gas tube attached to the upper handguard that is removable
does not take much pressure and does not touch the operating
rod: its sole function is to contain the gases. It still should be
cleaned out from time to time.
The bolt head should be cleaned thoroughly and crud brushed away
from the extractor. The firing pin should be free to move and not
crudded up.
The bolt recesses in the barrel trunnion should be clean and clear.
Lacquered case ammo may crud up the firing chamber, so use
a brush and solvent there on occassion.
AK is very forgiving, but it cannot and should not be totally neglected.