What's inside an oil filter and how does it work. A good oil filter will have an anti-drain back valve to help keep oil from draining back tonthe oil pan when the engine is shut off. It helps keep the filter and oil galeries full or oil and primed. The 2nd valve a good filter should have is a bypass valve. When the engine oil is cold it's too thick to flow thru the filter element, so rather than starve the engine of oil the pressure build up will unseat the bypass valve allowing oil to flow arou