Coconut oil has always been very good for me when I am having any type of major, excessive or chronic dryness and if my go-to jojoba oil doesn't work.
Dandruff is usually just very dry scalp which may occur from changes in the temperatures. Some scalps and hair will react to changes in the seasons by becoming dehydrated.
I think Lea has really covered most of the best tips, although Jamie's idea to use sesame seed oil is an excellent tip to try. I find sesame oil to be lighter. While coconut oil may be too heavy for some, sesame and jojoba can be a lighter alternative.
I have not tried Selson Blue, but if the dandruff is super challenging, it's worth a try for sure.