I've painted three sailboats and a couple of canoes over the years with Brightside (rolling and tipping) and never added a single drop of anything to it. They came out very nice. The first coat usually looks awful, but the second coat is gorgeous. The reason you add things to paint is because it isn't doing something you need it to do (not flowing, not leveling, brush dragging, etc.). The first thing to do is to try some and see how well it's working right out of the can. As for thinner, the general consensus is that the paint is so expensive these days, fixing a mess is such a pain in the butt, and modern paint chemistry is complex enough that saving a few bucks with non-factory thinner is usually more risky than smart.
Two coats of Brightside, painted out in my driveway. It had sat outside for three years after being painted and had spent the previous summer out on a mooring buoy when this photo was taken. It was in for spring cleaning.