For dry hair, the serum base that actually absorbs is coconut oil, jojoba, and a few drops of squalane. Castor oil, the ingredient most commercial dry-hair serums lean on, mostly sits on the surface, which is why so many of them leave fine hair looking unwashed an hour after application.
Absorption is chemistry, not marketing. Small lipids slip between cuticle cells; large, sticky ones stay outside and coat.
The oils with real absorption data
Coconut oil is the one plant oil with published penetration research behind it: a 2003 Journal of Cosmetic Science study found it passes into the hair shaft and reduces the protein loss that happens during washing. Our breakdown of coconut oil for hair and its penetration data covers what that work does and does not prove.
Jojoba behaves differently. It is a wax ester rather than a true oil, close in structure to human sebum, so it spreads thin and sinks in without the film heavier triglycerides leave behind. The details are in why jojoba is a wax ester, not really an oil.
A ratio you can mix at home
Two parts melted coconut oil, one part jojoba, three or four drops of squalane per ounce. Work it through damp hair from mid-length to the ends, then wash out after twenty minutes, or leave a couple of drops in on the very ends on dry days. Damp hair takes the blend better than fully dry hair.
Where castor oil fails dry hair
Ricinoleic acid, the bulk of castor oil, is a large and viscous molecule. On thick, coarse hair it works as a pre-wash seal; on fine or low-density dry hair it reads as grease within hours. Sealing has its place at the styling stage, and our piece on styling oils: sealing versus weighing hair down draws that line.
Serum will not fix bleach damage either. Oil masks the feel of porous hair for a day; it does not rebuild keratin. A bond treatment or a trim does that work.