The oil you reach for without thinking
Every formulator has a default oil: the one that goes into a first prototype before any thought has been given to it. In our formulators' experience that oil is usually oleic. Olive oil, high-oleic sunflower, hazelnut, avocado and, on the border of the group, argan share one dominant fatty acid, oleic acid, and they are chosen as liquid base oils more often than any other group because, in practice, they are the least likely to cause a surprise. This article, the second spoke of our series on plant oils by fatty-acid group, is about why that predictability exists, where the members of the group genuinely differ from each other, and the two situations in which a formulator can have too much of a good thing.
One double bond, and what it buys you
Oleic acid is cis-9-octadecenoic acid: an 18-carbon fatty acid with one cis double bond between the ninth and tenth carbons, C18:1 in the shorthand of a certificate of analysis. That single double bond does two things. It puts a kink in the chain that hinders packing, which is a large part of why oleic-rich oils are liquid at room temperature while the palmitic and stearic butters are solid. And it leaves the molecule comparatively resistant to oxidation: lipid oxidation begins with the loss of a hydrogen atom from a carbon next to a double bond, and the most vulnerable position of all, a bis-allylic carbon sitting between two double bonds, does not exist in oleic acid. Linoleic acid, with two methylene-interrupted double bonds, has one such carbon; alpha-linolenic, with three, has two. The double bond is not the only thing that decides an oil's physical state or shelf life, but that structural difference is the main reason the oleic group sits between the saturated fats and the polyunsaturated oils on stability tables.
The Codex Alimentarius standards give the reference ranges. Olive oil, under its own standard (CXS 33-1981), runs from 55.0 to 83.0 percent oleic acid, with linoleic at 3.5–21.0 percent and palmitic at 7.5–20.0. High-oleic sunflower oil, listed as a separate named product in the vegetable-oil standard (CXS 210-1999), carries 75.0–90.7 percent oleic acid, and high-oleic safflower 70.0–83.7. The iodine value, an analytical measure related to the degree of unsaturation, tracks the composition: 78–90 for high-oleic sunflower against 118–141 for conventional sunflower and 128–150 for grapeseed. A 2005 study in the Journal of Agricultural and Food Chemistry that oxidised purified olive and high-oleic sunflower oils in a Rancimat at 100 °C found that, at the same antioxidant concentration, the induction time fell as the unsaturation of the triglycerides rose, and that the Rancimat stability of olive oils depended mainly on triglyceride composition and antioxidant content. Under those accelerated conditions, then, stability followed the fatty-acid make-up and the antioxidants rather than the botanical name. That does not establish the shelf life of any particular oleic-rich oil or of a cosmetic made from it, which still has to be measured, but it is why the group as a whole sits where it does.
Who is in the group
Olive
The reference member, and the one with the widest legitimate range: a Codex-conforming olive oil can be anywhere from 55 to 83 percent oleic, and that range overlaps the high-oleic sunflower range, so the lot in front of you has to be judged from its own certificate of analysis rather than from the name. Olive is also the one oil in the group whose public standard grades it partly by free acidity: extra virgin olive oil has a free acidity, expressed as oleic acid, of not more than 0.8 grams per 100 grams, virgin not more than 2.0, ordinary virgin not more than 3.3, refined olive oil not more than 0.3 and the blend sold as "olive oil" not more than 1. In our experience virgin grades keep a marked colour and odour while refined olive oil is pale and close to odourless, so for a leave-on cosmetic the odour of a virgin oil is a formulation decision in its own right.
High-oleic sunflower
Conventional sunflower oil is a linoleic oil: a 2008 review of sunflower breeding in the Canadian Journal of Physiology and Pharmacology puts its typical profile at 16–19 percent oleic and 68–72 percent linoleic acid. High-oleic sunflower is a different material: the same review reports that induced mutations produced genotypes exceeding 90 percent oleic acid, and Codex treats high-oleic and mid-oleic sunflower oils as separate named products with their own composition and iodine-value ranges. In our formulators' experience this is the workhorse of the group: light in colour and odour, usually the least expensive oleic oil on a UK supplier's list, and the base most often chosen when a formula does not want the character of olive. The one thing it demands is attention on the certificate of analysis. "Sunflower oil" on an invoice can mean the conventional, mid-oleic or high-oleic product, and they behave differently in a formula; the oleic and linoleic percentages settle which one is in the drum.
Hazelnut
A 2017 study in Natural Product Research of two Italian cultivars and two wild types found oleic acid at 83.5 ± 1.0 percent of the fatty acids on average, with saturated acids as minor components, which places hazelnut with the most oleic-rich oils in that comparison. Formulators generally describe hazelnut as lighter in feel than olive, which is craft experience rather than a measured result. In our experience its price keeps it out of body-care bases and in facial oils and premium blends.
Avocado
Avocado oil comes from the flesh of the fruit rather than a seed, and its fatty-acid profile is dominated by oleic acid, with the exact content varying by cultivar, ripeness and origin, as a 2026 review in Comprehensive Reviews in Food Science and Food Safety sets out in detail. Its unsaponifiable fraction is a topic we treat in its own article later in the series; for the purposes of the oleic group, formulators use it as a rich, green base oil.
Argan, on the border
Argan oil is usually filed under "oleic" and usually described as light, and the composition explains why both labels are half right. A 2020 study of cosmetic argan oils in Molecules found oleic acid at 39.5–50.8 percent and linoleic at 27.1–39.3 percent. Roughly a third of its fatty acids therefore belong to the linoleic group. Formulators describe argan as lighter than olive, which is craft experience; and with more doubly unsaturated chains than a true high-oleic oil, it would be expected to oxidise faster, although that is an inference from composition, not a measured result from the cited study. Reading argan as a blend of two groups explains it better than either label alone.
What the group feels like
Ask formulators to describe oleic oils on skin and the words are consistent: rich, cushioning, slower to absorb than the linoleic oils, with a fuller residual film. This is craft experience, not a measured property, and it varies with everything else in the formula, but it is consistent enough to be useful. It is the reason oleic oils dominate body oils, massage oils, rich creams for dry and mature skin and cleansing balms, and the reason they are used sparingly in a mattifying gel-cream or a product for oily skin. When a client complains that a face oil "sits on the skin", the first question worth asking is how much of it is oleic.
Two places where oleic can work against you
The skin barrier
The oleic group's reputation as gentle and universal took a knock from a study that every formulator should know. Danby and colleagues (Pediatric Dermatology, 2013) ran two randomised, forearm-controlled studies in nineteen adult volunteers with and without a history of atopic dermatitis. In the second, volunteers applied six drops of olive oil to one forearm and six drops of sunflower seed oil to the other, twice daily for four weeks. Olive oil caused a significant reduction in stratum corneum integrity and mild erythema; sunflower seed oil preserved stratum corneum integrity, caused no erythema and improved hydration in the same volunteers. The authors' own conclusion was that topical olive oil significantly damaged the skin barrier in this setting and that its use for dry skin and infant massage should be discouraged; that is their recommendation, reported here, not a finding about any finished product.
The study is small, it tested neat oil applied repeatedly to adults, and it tested no infants and no finished products, so it does not condemn every formula that contains olive oil. What it does is remove the assumption that "natural" and "oleic" mean "barrier-friendly". One possible mechanism comes from pharmaceutical research, where oleic acid is a well-known penetration enhancer. A 2023 study in Molecular Pharmaceutics on ex vivo human skin found that free oleic acid at 0.75 percent fluidised stratum corneum lipids, sped the permeation of the model drug diclofenac diethylamine and adversely affected two indicators of barrier integrity, transepidermal water loss and skin electrical impedance; the study was funded by Haleon. It concerns the free fatty acid, and an oil is overwhelmingly esterified triglyceride, so the bridge from that experiment to a whole oil is not established. The honest position is that the mechanism is not settled. The practical position is the one that applies to every group in this series: these studies do not establish the safety or performance of a finished cosmetic, and a leave-on product has to be evaluated as a finished formulation for the skin and use it is intended for.
The heavy formula
The second failure mode is simpler. Because oleic oils are stable and forgiving, they tend to accumulate in a formula: a bit of olive for richness, some avocado for the story, a base of high-oleic sunflower because it is cheap. The result is a product that is technically excellent and sensorially wrong for the customer it was made for. Oily and combination skin, humid climates and summer ranges all argue for shifting the balance toward lighter groups or esters and keeping oleic oils to the share that stability actually requires. The oleic group is a base, not an active; it earns its place by carrying the formula, and it should be dosed like a base.
Reading a certificate of analysis for an oleic oil
- Oleic and linoleic percentages together. They place the oil in its group and, for sunflower and safflower, tell you which variety you have bought. A "sunflower oil" at 80 percent oleic is the high-oleic product; at 20 percent it is the linoleic one.
- Iodine value. For sunflower, 78–90 is consistent with the high-oleic product, 94–122 with mid-oleic and 118–141 with conventional oil under Codex; investigate any mismatch with the declared variety.
- Free acidity. For olive the Codex maximums by grade are 0.8 for extra virgin, 2.0 for virgin, 3.3 for ordinary virgin, 0.3 for refined and 1 for the blend sold as "olive oil" (grams of oleic acid per 100 grams). For the other oils, compare against the Codex maximum acid values of 0.6 mg KOH per gram for refined and 4.0 for cold-pressed and virgin oils.
- Peroxide value. Codex maximums: 20 milliequivalents of active oxygen per kilogram for virgin olive oils, 5 for refined olive oil and 15 for the blend; 15 for cold-pressed and virgin and 10 for refined oils under the vegetable-oil standard. A value near the maximum is a snapshot of primary oxidation at testing, not a forecast.
- Odour and colour against the grade ordered. A virgin olive oil that smells of nothing, or a "refined" oil that smells of olives, is a question for the supplier before it is a formulation problem.
Where the group belongs
| Format | Typical practice | What the chemistry and the studies say |
|---|---|---|
| Body oils and massage oils | The default base | Liquid, stable, low iodine value for a liquid oil; richness is welcome here |
| Rich creams and balms for dry, mature skin | Widely used as the main oil | Low unsaturation for a liquid oil; feel is reported by formulators as cushioning |
| Cleansing balms and oil cleansers | Common as a base alongside lauric oils | Low unsaturation; slip and cost are formulators' reasons rather than measured ones |
| Facial oils for oily or combination skin | Used at a minority share | Reported heavy feel; formulators shift toward linoleic oils or esters |
| Leave-on products built on olive oil | Evaluate the finished formulation for its intended use | Danby et al. (2013): neat olive oil, applied repeatedly to adult forearms, reduced stratum corneum integrity; sunflower preserved it |
| Carrier for actives | Common | Free oleic acid is a documented penetration enhancer (Kováčik et al., 2023); an oil is mostly triglyceride, so read across with care |
The oleic group will rarely be the interesting part of a formula, and that is its virtue: it is the liquid base the interesting parts are built on. Composition tells you what to expect from it; stability, compatibility, delivery and feel still have to be demonstrated in the finished formulation. Know its two weak points, the barrier question and the tendency to make everything rich, and it becomes the most dependable group in the cupboard.
Sources
- Codex Alimentarius. Standard for Olive Oils and Olive Pomace Oils, CXS 33-1981 (revised 2015): fatty-acid composition, free acidity by grade, peroxide value. FAO PDF
- Codex Alimentarius. Standard for Named Vegetable Oils, CXS 210-1999 (amended 2015): high-oleic sunflower and safflower composition, iodine values, quality factors. FAO PDF
- Relationships between oxidative stability, triacylglycerol composition, and antioxidant content in olive oil matrices. Journal of Agricultural and Food Chemistry, 53(14), 5766–5771, 2005. PubMed 15998146
- Škorić D., Jocić S., Sakač Z., Lečić N. Genetic possibilities for altering sunflower oil quality to obtain novel oils. Canadian Journal of Physiology and Pharmacology, 86(4), 215–221, 2008. PubMed 18418432
- Granata M. U. et al. Fatty acid content profile and main constituents of Corylus avellana kernel in wild type and cultivars growing in Italy. Natural Product Research, 31(2), 204–209, 2017. PubMed 27605244
- Overview of Factors Affecting the Fatty Acid Composition of Avocado Oil. Comprehensive Reviews in Food Science and Food Safety, 25(2), 2026. PubMed 41762643
- Bučar Miklavčič M. et al. Fatty Acid Composition of Cosmetic Argan Oil: Provenience and Authenticity Criteria. Molecules, 2020. PMC7570657
- Danby S. G., AlEnezi T., Sultan A., Lavender T., Chittock J., Brown K., Cork M. J. Effect of olive and sunflower seed oil on the adult skin barrier: implications for neonatal skin care. Pediatric Dermatology, 30(1), 42–50, 2013. PubMed 22995032
- Kováčik A., Kopečná M., Hrdinová I., Opálka L., Boncheva Bettex M., Vávrová K. Time-Dependent Differences in the Effects of Oleic Acid and Oleyl Alcohol on the Human Skin Barrier. Molecular Pharmaceutics, 20(12), 6237–6245, 2023. PubMed 37950377
