Oil weight (0w-40 for example), or viscosity, refers to how thick or thin the oil is. The temperature requirements set for oil by the Society of Automotive Engineers (SAE) is 0 degrees F (low) and 210 degrees F (high).
Oils meeting the SAE's low temperature requirements have a "W" after the viscosity rating for winter grade (example: 5W), and oils that meet the high ratings have no letter (example SAE 30).
Engines need oil that is thin enough for cold starts and thick enough when the engine is hot. Since oil gets thinner when heated, and thicker when cooled, most of us use what are called multi-grade, or multi-viscosity oils e.g. 0W-40, 5W-40. These oils meet SAE specifications for the low temperature requirements of a light oil and the high temperature requirements of a heavy oil.
A 0W-40, 5W-40 and a 10W-40 will all have the same viscosity at 210 degrees, typical operating temp of internal combustion engines. But the 5W and 10W will flow slower at engine start up than a 0W oil will. There is no disadvantage to using a 0W-40 but there can be with a 5W-40 and 10W-40 the colder the temp is.
Last edited by Miker; 07-04-2010 at 08:03 PM.