This actually worked for me. I had a few white dress shirts that had classic ring around the collar that refused to come out. I tried soaking them in all sorts of products that were supposed to remove these stains with no luck. They were ready to be thrown out.

I brought a large pot of water to a rolling boil on the cooktop. I placed three of my white dress shirts collars in the pot with the water boiling. Just the collars with the rest of the shirt hanging outside the pot. Mind you, I have an induction cooktop and I wouldn't suggest letting a shirt hang out unless you have one too. I let it boil for about three minutes then dropped the heat to just about boiling temp. I then added a half a scoop of Oxyclean and agitated the shirts by hand (obviously with a large metal spoon) for about five minutes. I then shut down the hob, put the cover on the pot and let the shirts sit in there for about a half hour.

I Took them out, rinsed them and there was absolutely no sign of dirt on the collars, none! I washed the shirts immediately thereafter and they look like the day I bought them.

I go the idea from this forum a while ago and was surprised to see it actually worked.