When I was a teenager I once went through a period of not washing my hair for several weeks. My hair was disgustingly greasy for the first two weeks, but during the third week the greasiness disappeared and my hair just seemed healthy. My hair had been restored to its natural state and it looked & felt great.
Face it, humans aren't 'designed' to wash our hair and it's only within the last century or so that it became commonplace for some people to wash their hair regularly. Most of our ancestors wouldn't have washed their hair... ever.