there are oil galleries and channels where oil thicker than the recommended viscosity will not pass resulting in wear and engine knock down in a couple of months ... some people use oil with "potato" mixture to suppress the blow while selling cars but in the end this reduces engine life like anything .. by the way, always feel and smell the engine oil when buying a car it tells you a thousand stories ... do this on three four cars and you'll know how a weak engine car oil smells like (it has the smell of a metallic grind)