What kind of SKS do you have? I have a Chinese that loves Chinese ball, but hates Wolf HP. On the other hand, I have a Yugo that loves both of those, but doesn't like Russian FMJ or Silver Bear. They both shoot cast lead well. Another Yugo I have doesn't like Silver Bear. What's a mother to do, with such picky children?
I'd recommend you to try a box of lots several different types, then stock up accordingly. I do for each of my "kids."
Just be careful with US commercial 7.62x39. They have softer primers, so your SKS may slamfire with them at the most embarrassing times. The firing pins in SKS's rattle free inside the bolt, and when the bolt slams forward upon returning to battery, the pin can set off sensitive primers. Milspec primers are designed to withstand this force, but US commercial are not.