I normally rub washing up liquid into the stain before washing. Those laundry soap bars are normally quite good too. Give a scrub with a little water and whatever pre treatment you use. I use bio powder too as the enzymes help remove stains plus powders contain bleaches that also help
The easiest way is to buy a cheap bottle of thin bleach and follow the dilution instructions on the bottle, soaking the polo shirts overnight, and then put them through a hot wash with non-bio powder and an extra rinse cycle to make sure everything is rinsed out. DD has eczema and this used to work for hers.
As much as eczema sufferers won't like this, biological washing powder will help. It has protease in it which will break down the stain. One option is to wash or pre-soak in bio and then wash as normal with your normal detergent.