What will remove cooking oil from a t-shirt?
I would start by pretreating the stain with something to help solubilize the cooking oil, breaking it up to make it easier to remove. Try liquid Clorox2® Stain Remover and Color Booster or liquid dishwashing detergent. Apply a little and rub it into the stain, and wait 10 minutes (but don’t let it sit too long or dry out on the fabric). How you proceed next depends on the item. Here are some options for different color types and fabric:
If the t-shirt is white and bleachable (avoid bleaching wool, silk, mohair, leather, and spandex) then wash it in the hottest water recommended on the care label using detergent and 3/4 cup Clorox® Regular Bleach2. Air dry and check for success – hopefully the stain will be gone. If a little remains you can repeat the treatment with a decent chance of eventual success; if the t-shirt has been kept out of a hot dryer.
If the t-shirt is colored, then wash in the hottest water recommended on the care label using detergent and more Clorox2®. Again, be sure to air dry the shirt and then check for success, and repeat the treatment if necessary.