How to Remove skin oil out of undershirts?

It stinks, but cheap cleaning ammonia is a great oil/grease cutter. Vinegar might work, too (but probably not together. try separately) The cleaner called "Simple Green" is also very good but not as cheap. I prefer the lemon version because I don't care for the smell of the original green version. Dawn dish soap is very well known for cutting grease in laundry applications. Put a little on the stain and then wash as usual.