Rub some dish detergent into the stain, preferably Dawn. Let it sit for a day or two. It will break up the oil and then you can machine wash. If it's a huge stain, you can also try soaking the shirt in vinegar and water, but the dish soap works better.
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Re: How do you get out an Aquaphor/Vaseline stain?
Nice! That's what I was thinking, but was too scared to try. Thanks!