For interior paint I prefer the semi-gloss, provides a very subtle sheen and is super easy to clean -no flaking, chipping, etc. I've painted a lot, and found that Sherwin Williams Ovation paint has been the easiest to use, and provides the best coverage. I even painted my ceilings with it and WOW! I was done it no time at all, with perfect coverage.
Back to the article. You can add water to all latex based paints / thinner to oil based paint. The tinting base has absolutely nothing to do with it. Say you are working outside and throughout the day you have to add a little water to keep the same consistency. If somebody really tried to add 20% to 50% water they no longer would be painting they'd be performing a whitewash or pickle finish.
A good paintbrush is key to a professional-looking finish. "A quality brush costs $15 to $25, but you'll discover that pros aren't as talented as you thought," says Doherty. "The equipment has a lot to do with their success." Most of our pros prefer natural-bristle brushes for oil-based paints, but they recommend synthetics for all-around use. When choosing a brush, pay attention to the bristles. Synthetic brushes are made of nylon or polyester, or a combination of the two. Poly bristles are stiffer, which makes them good for exterior or textured work, but for fine interior work, Doherty uses softer nylon brushes. Look also for tapered bristles, which can help you work to an edge, and flagged tips, which help spread the finish smoothly and evenly. Brushes are available in 1- to 4-inch widths. Most painters keep an arsenal on hand to match the job. "Use common sense," says Maceyunas. "A smaller brush gives you more control, but no one wants to paint a door with a 1-inch-wide brush." Doherty recommends starting with a 2- or 2-1/2-inch sash brush. The angled brush makes it easier to cut to a line and puts more bristles on the work than a square-tipped brush.