In article , Joe bristle, the best you can afford.
Quality brushes of any type, man-made or natural bristle work just fine.
Clean your brushes using whatever method is recommended by the paint manufacturer, it usually says on the tin.
Even if the recommended method is some sort of organic solvent, give a final clean with water and "fairy liquid" (or whatever liquid detergent you use for washing up (the old way, by hand) in your parts. Rinse thoroughly in clean running water and dry on paper towel.
Never leave brushes standing in a jar of solvent or brush cleaner. If they need to be left for a while for a brush cleaner to work (If you always clean properly you won't need to do this) they should be suspended so that the ends of the bristles aren't touching the bottom of the jar.