When a diesel oil breaks down, varnish and sludge deposits form, causing wear and robbing the engine of performance. In cold weather, thickened oil causes difficult starting as the engine fights to turn over against the semi-solid lubricant. Superior cold-weather performance means that a diesel engine doesn't have to waste fuel running all night in cold weather. It means that during starts and warm-ups, critical engine surfaces slide on a tough, reliable film of oil. Oil thickening and oil consumption occur when high temperatures evaporate the lighter, more volatile portions of the oil.
Heat also causes chemical reactions in the engine that form gummy sludges and varnishes.
Low volatility reduces oil consumption and improves fuel economy and engine protection.