Why does engine oil need to be thick (viscous)?
Has to do with thermal breakdown under the high engine temperatures and the outdoor temperatures. The lower number on say 5W30 means the oil is supposed to imitate 5 weight oil in cold weather, and 30 weight in hot weather. The higher the number the thicker the oil. Thin oil would or will burn off in the summer months when it is hot outside. The 5 weight oil flows easily in cold weather and reduces the risk of engine damage. The most engine damage occurs during the start up process and if the oil is too thick to pump, then your cylinder walls, piston rings, crank and camshafts all suffer from lack of proper lubrication. They KEY is to get quick lubrication going throughout your engine. My 2005 VW diesel uses Mobil 1, a synthetic oil, and it is more like water and stays that way. Makes for very easy starts in cold weather and reduces engine wear during the initial start up process. Diesel engines are tighter and have a much higher compression than gas engines.
