Hi Lucinda, nice to meet you, and welcome
The feedback I receive from my customers with oily and/or dandruff tendencies is that the gentle based shampoos work wonders for both conditions.
Using a less harsh detergent doesn't irritate the scalp to further produce oil, in order to keep the skin's acid mantle in order. When harsh shampoo based detergents are used, the skin produces oil as a defense mechanism, to protect it’s pH balance, which results in over oiliness = Oily Hair.
In drier scalp cases, the harsh detergents can just completely make the scalp very dry, and prone to flaking and itching (Dandruff). Using a less harsh detergent based shampoo will help to stop this from happening.
My usual customer's feedback suggests that it takes about a month to notice a difference, which does concur with how long the skin takes to form from the base to the outer layer.
Of course, there could be another explanation, stress/health related, but using harsh detergents are the most common and so should be ruled out first.
Finally, I normally recommend the powder cleansers for highly sensitive people, who just cannot use the mildest of natural mild detergent based shampoos. The reason for this is that it can be time consuming to create the mixture, and also, it can be quite a messy ordeal, especially when using the dark brown powder. But yes, the powder cleansers would be the best to use if someone was suffering from skin complaints, ie. to use them short term whilst waiting for the skin to normalise itself.
I hope this helps with your query?
Don’t hesitate to ask for more clarification, if you need more.