Cover it in washing-up liquid, work it in with a finger, leave it fifteen minutes, then wash as hot as the garment allows.
- Blot off anything sitting on the surface with kitchen paper.
- Sprinkle on cornflour or talc if the mark is fresh and heavy, and give it ten minutes to absorb the loose oil.
- Work washing-up liquid straight into the mark. Not detergent, not a stain bar: washing-up liquid is formulated to take grease off crockery and it does the same job on cotton.
- Leave it fifteen minutes, then wash at the hottest temperature the care label allows.
Grease is the one stain where heat is your friend, but only after treatment. Put an untreated greasy garment through a hot wash and the oil spreads and sets across a wider area than the original mark.
Grease will not dissolve in water, which is why rinsing does nothing. Detergents work by surrounding an oil droplet with molecules that have an oily end and a watery end, so the droplet can be carried away in the rinse. Washing-up liquid has more of that chemistry per drop than laundry detergent does.
Household chemicals can be dangerous in combination. Never mix them, keep the room ventilated, and test anything on a hidden patch first. Full disclaimer.