I always recommend against STP, unless you have a wornout engine and you want to get one more summer out of it. I’ve never used STP since my student days car (57 Plymouth, well worn) and have not overhauled an engine since the summer of 1964. Modern oils have all the aditives you need. Fuel system cleaners (the ones that work)are a good idea every 30-40,000 miles or so.