Well... STP oil treatment is often used when engine needs an overhaul and the owner cant afford one at the moment.
Normal oil losses its viscosity and lubrication at high temperatures so tends to get through weak piston rings and valve seals.
STP purpose is to increase the viscosity index of the oil and also stabilizes it on high temperatures thus giving a better ring to surface seal and may also reduce seepage from valve guides. So reducing the oil reduction and enhancing the compression (power) of the engine, BUT it is a temporary arrangement only coz thick oil also has its drawbacks like cold start problems, slow circulation through engine and oil galleries, etc.
Ultimately you have to overhaul the engine and when you do , DONT settle for semi overhaul (if you want to secure your expenses)
Cheers
