this thread is very entertaining, yes 5w30-40 is rated for certain temp range. Pours like 5w at -30f and lubes like 40 oil but what people are missing is it will leak through seals as it heats up to temp and flows like water, And the higher the mileage the worse thing get as far as seals go. Plus you start to get blow by on the piston rings as the oil does not have the vocasity to fill the micro gaps between ring and cylinder walls. This is not my words just a paraphrase from what is out there from oil companies and engine experts, not shade tree/DIY types.