Oil additive didn't cause that issue in 1500 miles.I have seen the same issue in Dodge,GM and import(Nissan)engines.I think its poor oil or lack of proper OCI. We cleaned many of them in the shops I have worked in.Pull pan,valve covers and start scraping and spraying parts cleaner from the top down.Clean the underside of the valve covers or more will flake off and clog the pickup screen.Use to have to soak the pickup screens in old type carb cleaner or send out with the valve covers and have vatted at a machine shop.Charged 8 hours labor plus material with NO WARRANTY.I don't remember any issues with any we cleaned up.Techs hated doing them.One nasty job.