For acrylic painters we recommend using synthetic brushes, particularly due to their chemical resistance. Acrylic dispersion paints have a high pH and the alkalinity damages natural hair, which makes them break easier. Acrylic dispersion paints also contain ingredients that strip natural hair of its natural oils, causing the hair to become brittle.
Both natural and synthetic brushes can be used for oil painting. Natural hog bristle brushes are great because they have good stiffness and can hold more paint than synthetic brushes could. Thus one brush load of paint goes a longer way and one is not required to pick up more paint all the time. Especially thicker impasto painting requires stiffer bristle brushes. For fine details and glazes, red sable brushes or soft synthetic hair brushes with good spring are great.