My scalp behaves similarly to yours - it doesn't produce much in the way of oil. Even after four or five days, my scalp itself is fairly dry, doesn't flake much, and what flakes are there are tiny and dry. If I were to scratch (very gently) at my scalp, there isn't usually anything under my fingernails. After four days, the roots behave as if they're a tiny bit greasy, but my scalp itself is generally dry.
Of course...I thought this meant that I was successful in stretching my washes, not that something was necessarily wrong! If your scalp seems healthy, aside from dryness, then maybe this is just how your scalp reacts to an SLS-free routine!
I wash my hair every 5 days. Before every wash, I do heavily oil my hair and scalp for a few hours before the actual washing. It seems to protect my hair from drying out too much and stops any minor itching I'm experiencing from my dry scalp.
I'm going to be spying on this thread to see if it turns out I've been doing something horribly wrong for a couple months now, heh!