“I looked into that but the TSP in my hardware store states: Contains no phosphates on the label.”
In Colorado, you can’t buy TSP anymore as it is supposedly used to make meth, and stores all “voluntarily” quit selling it. Instead, they sell a product that has the brand name “TSP”, which of course contains no TSP.
What you want as an additive to both clothes washing and dishwasher is STPP anyway. Real STP can damage your washer and dishwasher.
Because it's meant to be put in your crankcase.