In most firearms, grease lubricates very well and prevents wear better than thin oils.
Grease stays put and doesn't dry out, sling off, or run off.
More and more AR Match shooters are using grease in their rifles with excellent results.
Some people claim that grease collects grit and fouling and gums things up. This supposes that you shoot your rifle in a sand or dust storm and that you never clean it.
I know several AR Match shooters who shoot their rifles for an entire season by just adding more grease when needed and only clean the bore.
In the AK apply a thin coat of a good heat and water resistant grease to the "tail" of the bolt where it fits into the bolt carrier, the bolt locking lugs, the bottom of the bolt and bolt carrier, the receiver rails, the cam surface in the bolt carrier and any areas that show finish wear that indicates friction.
Most any grease will do fine. many people go to a hardware, automotive, or Walmart and buy a tube of Lithium grease.