A result of years of development, here is a synthetic brush that actually feels like a bristle brush!
Individual filaments have the extreme taper of natural bristle and are gently coaxed with heat to a natural curve. The result is a brush that is ideal for water-mixable oil paints, because these brushes retain their stiffness in water. They are also less susceptible to the ravages of acrylic paint. They cost a bit more than bristle, but may save you in the long run.
Note — Brush sizes are not standardized. The length and width of a brush of a given shape and size varies considerably by manufacturer. Since fine art brushes are handmade, expect some variation even among brushes from a given manufacturer.