Can I use the same brush for oil and acrylic painting?
The most important thing is to keep your brushes clean and dry between uses. Use a mild soap and lukewarm water (marseille soap is my favorite) to clean your brushes after you have used them, whether acrylic or oil. Rinse them in warm water, flick the water out and then dip them in acetone. The acetone will dry out the oils, turps and waters, leaving your brush nice and dry.

I should say however, that not all brushes are created equal and not all surfaces are forgiving. A good rule of thumb is that if the ends of the individual hairs are splitting, you can groom the ends, but sometimes it’s just time to get a new favorite brush.