When painting with a thick or viscous paint such as oil or acrylic paint, the general rule is, the thicker the colour, the stiffer the brush needs to be. If you are using oil paint, of a heavy body acrylic, you need a brush with enough resiliance to enable you to manipulate the colour with complete control. If the paint has been thinned, or you are using a soft body or fluid acrylic paint, you would need a softer haired brush.
In general, a bristle or stiff synthetic brushes are perfect when using heavy colour right out of the tube. If you thin the colour down, a medium to soft hair filament should be just right. If you thin the colour to a more fluid consistency, then it is best to go for a soft synthetic or natural hair brush.