I use whitening products that you just rub into the stain pre-wash and then leave to soak for a bit. I can't remember the name, but they have TV ads all the time so shouldn't be hard to find the synthetic stain remover I'm thinking of.
Just check whether the stain remover mentions on its box which synthetic materials you can use it on, some stains on synthetic clothing don't come out very easily, especially poor quality synthetic material where the stain absorbs almost at once.
