Make a paste with Borax and water, and rub into the stained areas. I use Borax to clean greasy pots and pans, and oil and butter spills in the kitchen, so I don't see why it wouldn't work on your shirt. I also make my own laundry detergent, and Borax is one of the ingredients.
Fels Naptha is also a great stain remover.
Whatever you use to treat the stain, wash it in cold water and air dry it. Do NOT wash it in hot water or use the dryer--you will set the stain. It may take a few treatments and washes to get the stain completely out.