I'm not sure, but MAYBE there's some carry-over value in some experience I had making soap. I have not much hair to which to apply this.
If you make it yourself, you can leave as much residual oil in the soap as you want.
Basically you add sodium hydroxide to basically whatever fats and oils you have. There are online calculators to make it really simple. The sodium hydroxide will get used up, and a certain percentage of unreacted fat will be left over (you certainly don't want any unreacted drain opener in there).
The more there is left the more "moisturized" you will feel upon rinsing. That, and all the glycerin remains, as opposed to being skimmed off and sold as a separate product.
Just a thought. I can vouch for how much nicer my skin felt in the dry winter months in particular. No ashy look or feel despite vigorous scrubbing.
Plus it's all earthy and crap to make your own.