Paint that's based on
oil, usually used for
exterior painting since it lasts longer than
latex-based paint. To clean a brush with oil-based paint on it use
paint thinner.
Gasoline is cheaper but you can pour some paint thinner in a
coffee can, wash the brush, and throw the dirty paint thinner in a bucket. Eventually the paint will settle to the bottom and you can reuse the paint thinner at the top.