That greasy feeling and appearance you see at the scalp is called sebum. Sebum is a waxy, oily substance produced by our sebaceous glands. Actually natural sebum is good. If glands are not clogged sebum is produced coating root hairs on a consistent basis. Different hair types experience sebum production differently. A dry, thick haired person craves sebum, we want shine and to be tamed (I’m including myself in this). A thin, fine haired person struggles with sebum, after a few days without washing, hair flattens and lays too flat against scalp.
A good discovery is learning which days hair performs best after sebum production occurs, that discovery will determine how quickly a hair wash is needed.
Hope this helps, I am here to help you learn and correct more issues with your hair.