AK's aren't very particular. I've used WD 40, Liquid Wrench, rubbing alcohol...sorta' whatever is handy.
As a side note...I got a couple of nasty old 91/30's ($55 for the two). The bore looked like there was something growing in it. It turned out to be very, very, very old cosmo. I got them to the point where they were safe to fire using "Bore Scrubber". I found the stuff in Wal Mart by the Hoppe's No.9. It is some evil smelling stuff, but it is a bit more effective than good ole' Hoppe's. I also found that wrapping steel wool into the bore brush helped a lot. Still, the last residue would not come out and the bore was quite dark. I would shoot a few rounds (so the bore was hot) an spray WD-40 down it, let it cool and clean it again with the steel wool impreganted brush. After a few times the bore in both guns looked bright and perfect.