Yeah, over a period of years and years. But to a new boater, who just put his life savings down on a boat, it makes for a long winter thinking about how your engine will blow up in the spring because the oil wasn't changed in the fall. Been there.It's more like contaminates (carbon, oxidation etc.) that strip down the integrity of the base oil and can act like a mild acid. I am speaking from a two stroke diesel perspective, but the same exist's for all engines. It's not a wives tale, it's real. But you need to understand it isn't fast acting, but it does exist and will shorten the life of your engine.