It could be a problem at your end, but it also could be a problem in manufacture: sometimes the sewing machine will drip oil onto the garment. While I don't recall hearing about it with Gildan, it is a problem that all t-shirt blank companies potentially face (if it's not caught in quality control). I recall a while ago there was a brand of kid's blanks that someone was having a lot of trouble with.
Is it possible that the oil becomes more noticeable after the dryer, but is there before? I'd check over incoming stock to make sure it's not happening before they get to you.
Is it possible that the oil becomes more noticeable after the dryer, but is there before? I'd check over incoming stock to make sure it's not happening before they get to you.