Using a Makita 18v lithium ion driver/drill to put these screws in. I will try wd40 or candlewax. So what size pilot hole should I use?
What kind of wood are you using that can stall out an 18v Makita?
I had some old southern yellow pine studs that I couldn't get #10 four-inch screws into without pilot holes. IIRC, pilot holes for any mechanical fastener in wood can be 90% of the diameter of the fastener without compromising the strength -- just pick a bit by eyeball. Or you can buy bits sized for screws and use the bit for the size of the screw. #8 or #10 for 3-inch (depending on if they're #8 or #10 screws).
You should use decking screws, not drywall screws.
You can also drill a full sized pilot through the top piece. I might do that if I was working in oak, for instance, especially near the end of a small piece of wood.
I get my screws at Lowes and Home Depot. I don't know if there are better screws available, or what would make them better. The ones I get go in and hold the wood together, and that's all I need them to do. Occasionally (a few times in a five-pound box) I break them off as they're going in. Usually it's because I'm running them into a knot, or that old southern yellow.