Z Premium 10W40 Engine Oil, All our branded optimum quality products are safe to use, reliable, having high functionality and longer shelf life. The lower the "W" number the better the oil's cold temperature/cold start performance. The 40 in a 10w-40 simply means that the oil must fall within certain viscosity limits at 100 C. ... Once again the lower the number, the thinner the oil: a 30 oil is thinner than a 40 oil at 100 C etc.
