You shouldnt assume you can use a 0w-20 where a 5w-20 is specified, especially if the engine's sump does not run at or above boil. Looking at the viscosity curve and the 0w should (if a true 0w) present a flatter curve with increasing delta as you move away from the running viscosity. The 0w could present a substantially lower viscosity than the 5w. If the specific engine design and its manufacturing tolerances present loads that exceed the film strenght and boundary lubrication of the lubricant, you will have obvious mechanical noise, increase wear, ring chatter, increasing fuel dilution and subsequent failure. I am no troll- I would say knee jerk, blanket responses without understanding the physics and application are the most dangerous.