In the mid-2000s, Carmen Tal, who was born in Chile and lived in Montreal, had hair that was badly damaged from years of processing. At a salon in Tel Aviv, a stylist treated it with an argan oil product, and the difference was big enough that she brought it home to Canada. She and her husband, Ofer, went on to found Moroccanoil around it, bottled it in amber glass, and hairstylists were the first to catch on. Moroccanoil Treatment is still in the first grid, and it is where I would start.

The thing I wish someone had told me earlier is that hair oil means two completely different products. One is a finishing oil for your lengths: a couple of drops on damp or dry hair to smooth frizz, add shine and, often, protect from heat. The other is a scalp oil, something you massage into the roots and leave on for a while before you wash. Different ingredients, different jobs, and using one in place of the other is how people decide oil makes their hair greasy.

So the first grid is finishing oils, for shine, frizz and heat. The second is scalp and pre-wash oils, the Sunday-night kind.

Here is a test I like: put two drops of a finishing oil on dry ends only, never the roots, and look again the next morning. If your hair looks shiny and not stringy, you have the right amount.

Finishing oils go on the lengths and ends, never the roots. Rub one or two drops between your palms, then smooth them over the ends, either on damp hair before styling or on dry hair to tame flyaways.

Moroccanoil Treatment is the original, an argan oil treatment that smooths frizz, detangles and protects from heat, and the brand says it boosts shine by up to 118 percent.

If you use hot tools, Gisou, OUAI and Kérastase all list heat protection up to 450°F. Gisou's is built on Mirsalehi honey to hydrate and boost shine. OUAI's is ultra-lightweight and color-safe. Kérastase's Elixir Ultime, with camellia, is the richer all-rounder for every hair type, and it comes in a refillable bottle.

If your hair is damaged, Olaplex No. 7 and K18 are the repair-minded ones. No. 7 is a weightless styling oil with heat protection that reduces breakage and frizz and keeps color vivid. K18's oil uses the brand's own peptide to strengthen and repair damage while smoothing frizz.

If your hair is fine and oil tends to weigh it down, Verb Ghost is weightless, with moringa and argan, and Oribe's Gold Lust is light and absorbs quickly, adding silkiness without heaviness.

Here is a pairing I like: a finishing oil on damp ends before you blow-dry, then half a drop on dry ends to finish.

What I would skip: oil on the roots of fine hair on a day you are not washing. It will look like a third-day blowout by lunch.

The scalp oils are the slower, older ritual. The word shampoo entered English in the 1760s from the Hindi champo, to press or knead, and it first meant a head massage with oil. indē wild calls its oil Champi, the Hindi word for that massage.

Most of these go on dry roots and sit before you wash. indē wild's Champi is lightweight, with castor oil, hibiscus and rosemary, and it is made to reduce shedding and soothe dryness. Fable & Mane's HoliRoots nourishes the roots and strengthens hair before you shampoo. JVN's pre-wash uses rosemary and turmeric to clear buildup and soothe an itchy, flaky scalp. Act+Acre's Scalp Detox Oil is a 20-minute pre-wash with baobab oil that breaks down buildup and is made for a dry scalp.

Two are leave-in serums instead. Fable & Mane's SahaScalp is a weightless serum with amla for a dry, itchy scalp, gentle enough for sensitive ones. Guerlain's Abeille Royale is a non-greasy oil serum you can use daily on dry or damp hair, made with honey and royal jelly, and it is the luxurious one.

Then the simple oils. Wonder Valley's rosemary oil is light and works from roots to ends, adding shine and helping with split ends. Philip B.'s Oil is the brand's first-aid treatment for very dry, damaged hair, with essential oils and plant extracts.

Here is a routine I like: on Sunday, part your hair into sections, give yourself a scalp massage with a pre-wash oil for a few minutes, leave it for twenty minutes while you read, then shampoo twice.

What I would skip: a heavy pre-wash oil the morning of a big day. Leave time for that second shampoo.

I think hair oil is the easiest way to make hair look cared for, and the easiest to overdo. Less than you think, every time.

Start with one drop of a finishing oil on your ends and add another only if they still look dry. Save the scalp oils for a slow evening, when you can leave them in and wash them out properly. Get those two habits right and one small bottle lasts for months, and your hair looks better for it.