I'm still finding that Ballistol is still my favorite product to clean and rejuvenate G-10 handles. But I am watching this thread intently because if I ever find anything that works better than Ballistol I will switch to it ASAP. But the almost new, factory fresh look that Ballistol gives it works better than anything I've tried up till now.
Now for general cleaning of really greasy, oily, grimey type dirt I've had some good results with Bar Keeper's Friend cleanser.