By ingredient · Sub-chapter 04
Penetrating oils, sealing oils, and the ones that do both — sorted by molecular weight, use case, and what the hair science actually says.
102 how-to's · Updated 2 May 2026 · Avg. 4 min per piece · Edited by Nelly · Beauty & Style Director
Editor's note
Not all oils behave the same on hair. Some penetrate the cortex, travelling through gaps in the cuticle to reduce protein loss and improve internal moisture retention. Most don't — they sit on the surface as a temporary, occlusive film. Whether penetration matters to you depends on what problem you're trying to solve. Molecular weight is the deciding variable.
Light Oils
Light oils are characterised by low molecular weight and a higher proportion of unsaturated fatty acids. They spread easily, feel weightless on the hair, and are the only oils that evidence supports as genuinely penetrating the cortex — particularly coconut, olive, and avocado oils. Inside the cortex, penetrating oils bind to the protein matrix, reduce hygral swelling, and can measurably decrease protein loss during wet combing. Coconut oil is the most studied and most recommended for pre-wash treatment across all hair types.
Heavy Oils
Heavy oils — castor, shea, jojoba (technically a wax ester) — have higher molecular weights and are largely or entirely surface-active. They coat the outside of the hair shaft, providing a temporary seal that slows moisture loss, smooths the cuticle for shine, and reduces friction between strands. The buildup concern is real but manageable: a clarifying wash removes it. Castor oil's growth claims are anecdotal; jojoba's chemical similarity to sebum makes it the best scalp oil in the category.
Choosing
The question is not which oil is best — it's which oil for which purpose. Pre-wash treatment: light penetrating oils. Sealing after leave-in: heavy oils as the final LOC layer. Heat protection: coconut or argan, applied thin and absorbed before heat. Shine finishing: argan or squalane, one drop maximum. Scalp: jojoba first. The most common mistake is using too much of any oil.
Other ingredients sub-chapters
Everything we've published on hair oils
- Coconut oil: the evidence, all of it
- Argan oil vs Moroccan oil: the same thing?
- Castor oil for hair growth: what the evidence says
- Pre-wash oil treatment: who it's for
- LOC vs LCO: which order for your hair type
- Jojoba: why it's technically not an oil