We used to sell a lot of STP to people with old worn out engines. It quietened them down, cut down on oil consumption and raised the oil pressure. We even used it in our old GMC gassers and 855 Block Cummins engines to make a few more trips until we overhauled them. It needs to be added slowly with the motor hot and running. Otherwise a blob of it will settle in front of the oil pickup and starve the engine for oil. I think it has kind of outlived its usefulness, but it kept a lot of old engines going a little while longer back in its heyday. I still use ZDDP on flat tappet engines. My father sold an oil that had zinc in it. It was something that was developed during the war and was way ahead of its time.
