I don't guess putting grease would hurt where Ursae mentioned, but in my experiences (with a well built AK with proper Communist Block mil-spec heat treated parts) simple oiling will prevent ALL discernable wear.
Just beware of circumspect U.S. made receivers and other parts that aren't heat treated properly. In such case you would be well advised to wisely choose a Moly fortified Extreme Pressure lubricant of some sort. I've also seen some of the cheaper post-commie era gun parts (a-la Romanian) put on the U.S. civilian market with improperly heat treated bolts, etc.
Its hit or miss with the AK's we are allowed to have here based on the BATF's whims. But if you get a good one with all the proper mil-spec parts, then the [censored] thing will run forever, and flawlessly too! Just keep an eye on the shiny wear spots I mentioned. You'll notice if you've got a "soft" part in there. It will wear away a little every time you take it shooting. The good ones just turn silver and basically STOP wearing.