As the title says, what do these numbers mean? I always thought that the higher the number the thicker the oil and the thicker the oil gets the more protection (oil film) it provides. For example 0w30 provides a very watered down layer of protection vs. thicker oil such as 20W50. I also understood that the hotter the engine gets the thiner the oil gets! Based on these assumptions I figured using 20W50 in summer and 10W40 in winter is best setup for Vw engines. Is this correct?