Every curly-hair shelf is half full of bottles labelled "hair oil" and almost none of them tell you the one thing that matters: does the oil enter the hair shaft, or sit on top of it? That single distinction — penetrating versus sealing — decides whether an oil moisturises your curls or just makes them shiny. This guide maps the twelve most common oils by molecule size, matches each to a curl type and porosity, and gives you a damp-versus-dry protocol so the oil actually does what you bought it for.
Penetrating vs Sealing Oils
Which oils penetrate the hair shaft? Coconut, argan, olive, avocado, and palm oils contain fatty acids small enough to slip past a raised cuticle and into the cortex of the hair. Most other oils — jojoba, castor, grapeseed, almond, and the rest of the shelf — have molecules too large to enter; they form a film on the surface instead. The penetrating four reduce protein loss from inside the strand. The sealers reduce water loss from outside it. They do completely different jobs and they're not interchangeable.
The Society of Cosmetic Chemists describes the mechanism plainly in Conditioning Agents for Hair and Skin (2nd ed., Routledge, 2017): a fatty acid's ability to penetrate keratin depends on its molecular weight and its affinity for the protein structure. Lauric acid (the dominant fatty acid in coconut) sits at the small, high-affinity end of that scale. Oleic and ricinoleic acids — the bulk of olive, castor, and most others — sit at the large, surface-only end.
Knowing which side of that line your oil is on tells you when to apply it, how much to use, and whether it pairs with water or with cream. If you only take one idea from this article, take that one.
The Molecule-Size Table
Here are the twelve oils most commonly found on curly-hair shelves, ordered by the molecular weight of their dominant fatty acid. Anything in the top group penetrates; anything in the bottom group seals.
| Oil | Dominant fatty acid | Approx. MW (g/mol) | Verdict |
|---|---|---|---|
| Coconut | Lauric (C12) | 200 | Penetrating |
| Palm kernel | Lauric (C12) | 200 | Penetrating |
| Babassu | Lauric (C12) | 200 | Penetrating |
| Argan | Oleic + linoleic blend | 282 (mixed) | Penetrating (partial) |
| Avocado | Oleic (C18:1) | 282 | Penetrating (partial) |
| Olive | Oleic (C18:1) | 282 | Penetrating (partial) |
| Almond (sweet) | Oleic + linoleic | 282 | Sealing |
| Grapeseed | Linoleic (C18:2) | 280 | Sealing |
| Jojoba | Wax ester (C20–C22) | 600+ | Sealing |
| Castor | Ricinoleic (C18:1-OH) | 298 | Sealing (high-viscosity) |
| Shea butter (liquid fraction) | Stearic / oleic | 284 | Sealing |
| Mineral oil | Hydrocarbon chain | 350–500 | Sealing |
The penetrating four (coconut, argan, olive, avocado) come up again and again in the cosmetic-chemistry literature because they consistently outperform sealers on protein-loss tests — Schueller and Romanowski summarise the evidence in It's OK to Have Lead in Your Lipstick (Brain Pickings, 2011). Argan and olive are "partial penetrants" — some fatty-acid fraction enters, the rest remains on the surface, which is why they feel less drying than coconut on protein-sensitive hair.
The Penetrating Oils (Coconut, Argan, Olive, Avocado)
Coconut oil is the most-studied penetrating oil in cosmetic chemistry. Its lauric acid content (around 47%) gives it the smallest molecule on the shelf, and it reduces protein loss measurably when applied pre-wash. Is coconut oil good for all curls? Not universally. Lauric acid penetrates so readily that it can bind tightly to existing keratin proteins, which on protein-sensitive hair (often high-porosity, fine-strand, or recently protein-treated) leaves the strand feeling stiff or straw-like. Patch-test on a small section for a week before committing.
Argan oil is the safer first try for most curl readers. Its oleic-linoleic blend penetrates partially, smooths the cuticle, and rarely triggers the stiffness coconut can cause. Use a few drops on damp mid-lengths.
Olive oil is the workhorse pre-wash treatment in the Davis-Sivasothy and Massey traditions. Warm slightly, apply to dry hair an hour before shampooing, rinse out. It costs almost nothing and out-performs most "luxury" pre-wash treatments on protein-loss reduction.
Avocado oil is the heaviest of the penetrating four. Better suited to thick, coarse, high-porosity hair than to fine 2A/2B strands, which it can flatten.
The Sealing Oils (Jojoba, Castor, Grapeseed, Almond)
What's a sealing oil? A sealing oil has fatty-acid molecules too large to enter the hair shaft; instead they form a thin protective film on the cuticle surface that slows moisture loss. The sealing group includes jojoba, castor, grapeseed, and almond — and they're best applied after a water-based moisturiser, as step two in the LOC (Leave-in, Oil, Cream) or LCO (Leave-in, Cream, Oil) routines. Apply a sealer to dry hair without water underneath and you've trapped nothing in.
Jojoba is technically a wax ester rather than a true oil — its molecules most closely resemble human sebum, which is why it absorbs into the scalp without clogging follicles for most users. It's the lightest sealer and pairs cleanly with fine curls.
Castor oil (especially Jamaican black castor oil) is the heaviest, stickiest sealer on the shelf. Ricinoleic acid is the unique fatty acid here — it gives castor its viscosity and its reputation for slowing breakage at the ends. A pea-sized drop is enough; more and the hair turns into a wet rope.
Grapeseed is the lightest sealer after jojoba — high in linoleic acid, almost imperceptible on fine curls, and the right pick if you tend to over-apply.
Sweet almond oil sits between grapeseed and jojoba in weight. A useful pick for medium-density 3A and 3B hair that needs definition without weight.
A note on comedogenicity: coconut, almond, and shea-based products score moderate-to-high on the comedogenic scale, meaning they can clog scalp follicles in users prone to seborrhoeic dermatitis or scalp acne. The British Association of Dermatologists' seborrhoeic dermatitis guidance recommends limiting high-comedogenic oils at the roots in affected scalps; keep penetrating oils to the lengths and use jojoba (low-comedogenic) at the scalp instead.
How to Match Oils to Curl Type
Coil shape determines how much weight your hair tolerates before it loses pattern. The matrix below is a starting map — refine against your porosity (next section) before committing.
| Curl type | Best penetrant | Best sealer | Avoid |
|---|---|---|---|
| 2A / 2B | Argan (drops only) | Grapeseed | Coconut, castor, avocado |
| 2C / 3A | Argan, olive | Grapeseed, jojoba | Castor at lengths |
| 3B | Argan, olive, avocado | Jojoba, almond | — |
| 3C | Olive, avocado | Jojoba, almond, light castor | — |
| 4A / 4B | Coconut, olive, avocado | Castor, almond, shea | — |
| 4C | Coconut, olive, avocado | Castor (heavy), shea | — |
The pattern: looser curl types want partial-penetrants in tiny doses and the lightest sealers; tighter coily types tolerate (and often need) the full penetrating-plus-heavy-sealer pairing. This article on how to choose curly hair products explains the weight-class logic in detail.
How to Match Oils to Porosity
Porosity matters more for oil selection than for almost any other product category. The cuticle gap on high-porosity hair is wide enough to admit even larger molecules; low-porosity hair rejects most oils on the surface.
Low porosity — cuticle lies flat. Most oils sit on top and never absorb. Stick to the smallest molecules (coconut, argan) and apply warm — gentle heat lifts the cuticle just enough to let the oil enter. Heavy sealers will pile up and feel greasy without conditioning. Our low porosity curly hair routine covers the warming step in detail.
Medium porosity — the broadest tolerance. Almost any oil works; the limiting factor becomes curl type and strand width.
High porosity — cuticle is raised, the strand is thirsty and leaky. High-porosity hair loves penetrating oils (they fill the protein gaps) and needs sealers urgently to lock in moisture. LOC sequencing is essential here: water-based leave-in first, then the penetrating oil, then a heavy sealer to trap everything in.
The EU CosIng database (European Commission DG GROW) is a useful reference if you want to verify what's actually in a product before applying it to porous hair — the INCI name appears there for every cosmetic ingredient sold in the EU, with its function class listed. Bottles labelled "hair oil" often hide silicones or mineral oil under generic marketing copy; CosIng tells you the truth.
Application Method — Damp vs Dry, Scalp vs Lengths
The single most common mistake is applying oil to dry hair and expecting it to moisturise. Oil does not moisturise. Water moisturises; oil seals or penetrates. Sequence matters.
Penetrating oils — apply to damp hair (or pre-wash, to dry hair, then rinse off after an hour). Damp application lets the oil ride the moisture into the cuticle. Two to four drops for fine 2-3 hair; a teaspoon for dense 3C-4C. Always work through mid-lengths to ends, never the scalp first.
Sealing oils — apply to damp hair AFTER a water-based moisturiser. This is step two in LOC (Leave-in, Oil, Cream) or step three in LCO (Leave-in, Cream, Oil). Without water underneath, the sealer traps air, not moisture.
Scalp oiling — only jojoba and (sparingly) argan should touch the scalp regularly. Coconut and shea are comedogenic; castor at the scalp is fine for hairline-edge growth treatments but heavy weekly use can build up.
Hot-oil treatments — warm 1-2 tablespoons of olive or coconut oil to comfortably warm (not hot) skin temperature, apply pre-wash, leave for 30-60 minutes, shampoo out. This is the highest-impact penetrating application for high-porosity hair and integrates cleanly with the deep conditioning curly hair routine.
A founder note. When I first started using oils, I bought every bottle on the shelf and rotated them weekly because the curly community on Instagram seemed to. After a year my fine 3A hair was permanently flat, and three weeks of stripping back to one argan-oil bottle (literally one) brought the bounce back. The lesson we kept relearning at Zenvy: oil discipline beats oil variety. Pick one penetrant and one sealer, run them for a month, then judge.
Hair Oils FAQ
Which oils penetrate the hair shaft?
Coconut, argan, olive, avocado, and palm — these contain fatty acids small enough to enter the hair cortex. Coconut (lauric acid, ~200 g/mol) is the smallest and most-studied. Most other oils (jojoba, castor, grapeseed, almond) have molecules too large to penetrate; they form a protective film on the cuticle surface instead.
Is coconut oil good for all curls?
Not universally. Lauric acid penetrates readily but can bind to keratin protein in ways that stiffen protein-sensitive hair — often high-porosity, fine-strand, or recently protein-treated. The result feels straw-like rather than moisturised. Patch-test on a small section for a week before committing to coconut, and consider argan as a gentler partial-penetrant alternative.
What's a sealing oil?
A sealing oil has fatty-acid molecules too large to enter the hair shaft; instead it forms a protective film on the surface that slows water loss. Examples: jojoba, castor, grapeseed, almond. Apply sealers after a water-based moisturiser (the "O" step in LOC or LCO routines), never to bone-dry hair on their own.
Which oil is best for low porosity hair?
The smallest molecules — coconut and argan — and apply them slightly warmed. Low-porosity cuticles lie flat and reject most oils on the surface; gentle heat lifts the cuticle just enough to admit the smaller fatty acids. Heavy sealers like castor will pile up and feel greasy on low-porosity hair without conditioning.
Can I use hair oil every day?
Penetrating oils, yes — small amounts on damp ends are fine daily. Sealing oils, no — daily use builds up and weighs curls down. Once or twice a wash week is the right cadence for sealers. For more on weekly rhythm, see the Zenvy curly hair ingredients glossary.
Are mineral oil and silicones the same as natural oils?
No — and reading the INCI label matters. Mineral oil is a petroleum-derived hydrocarbon that seals well but never penetrates; silicones (dimethicone, cyclomethicone) are synthetic polymers that mimic sealing. Neither is harmful, but they're not interchangeable with natural penetrating oils. The EU CosIng database lists the function class for every ingredient if you want to check before buying.
If you want to skip the research and pick a starting kit, the Zenvy curly hair collection is sorted by porosity and curl type. Filter to your profile, pick one penetrant and one sealer, and run the routine for a month before changing anything. Not sure of your type yet? The Zenvy AI Curl Identifier maps a photo to one of nine patterns and flags your likely porosity in the same scan.
