Try dawn dish soap, for many years I had to wear white uniforms, put it on the spots, then stuff it into a ziplock bag. Let it sit over night, rinse in warm, not hot water come morning. Most oil stains should be gone. Repeat if not.
If the fabric is polyester, and it has gone through a laundry cycle already, been washed and dried with heat, the oil stain may have already set.
Have you tried washing the stain out yet, or is it a fresh stain?
Lisa, I would cover the whole stain with dry baking soda. Let it dry overnight, then shake the baking soda into the washing machine. Check the stain. If it or it’s outline is still there, make a scrubbing paste from 1/4 cup baking soda, 1/4 cup of water, a teaspoon of lemon juice without pulp, and 1 teaspoon of Dawn liquid. Stir the ingredients, then rub some of the paste on the stain, from edge to edge. Keep leftover paste for other stains. If this were not a white tank top, I would use white vinegar instead of water, as on a white apron, for instance. When the paste has been on the stain for several hours, rinse the tank in cold water, then wash the tank with other white clothes. This is your best chance of wearing the tank without stains. I hope one of these methods works. Jewell
Thank you All, I appreciate it 💙☺