Proper lubrication is crucial to maintaining a long, trouble-free service life for the engine. The primary component lubricating engines is oil. Oil comes in many different specifications, weights and grades. We’ll look specifically at some of the similarities and differences between 0W-40 and 5W-30.
First, it’s important to understand what the viscosity grading numbers mean. The “5W” or “0W” is the relative viscosity of the oil at a cold temperature. The higher the number, the thicker the oil. Multi-grade oils, like 5W-30 or 0W-40, change viscosity based on the operating temperature. The first portion, 5W or 0W, is the measurement of viscosity when the oil is cold (at 0 degrees F) and the second portion is the viscosity when the oil is warm (at 100 degrees F).
Engines are designed by engineers to operate with a specific viscosity. It isn’t recommended to deviate from the recommended oil viscosity, because all of the machined surfaces in the engine are specified for a certain thickness and size of oil molecule.
5W-30 is a very common oil weight for vehicles operating in North America. It’s relatively thin cold and warm viscosity make it ideal for operating in moderate temperature climate. There are many different tiers of quality, from older conventional petroleum base stock to full synthetic oils. Because of this variation, it’s important to make sure that the oil is a good quality. Older conventional style petroleum base stocks have more molecule size variation, which makes them less able to flow into very tight engine clearances. The process of making a synthetic oil is largely about refining the oil further until the molecule size is more uniform. This means that a 5W-30 that is a full synthetic oil will offer far superior engine protection to a 5W-30 that is conventional.
0W-40 is a less common, often specified for newer gasoline direct injection engines or light-duty diesel engines. It is only available in full synthetic. To get oil molecules that will flow easily 0 degrees F, a very uniform molecule size must be achieved, and this can only be done with synthetic motor oils.
One of the advantages of a 0W-40 full synthetic is because it is thinner when it is cold, it will provide better lubrication to critical engine surfaces when the engine is cold, at the time most engine wear occurs. However, the 5W-30, with slightly thinner high temperature operating viscosity will actually provide slightly better fuel economy over the life of the engine. The fuel economy improvement from a thinner oil is very minimal percentage, but when added over the course of several thousand hours of engine run time, can make a noticeable difference. In fact, many car manufacturers have been moving to thinner oil viscosity to help improve fuel economy.
Given that 0W-40 can be used in diesel applications, it is very important that the second viscosity number, the “at operating temperature” viscosity be slightly thicker. This is because the hottest parts of the engine, the piston rings, are almost twice as hot in a diesel engine as they are in a gasoline engine. A piston ring area on a gasoline engine runs between 250-350 degrees F, and a piston ring area on a diesel can be as hot as 600 degrees F. A thicker oil is required because as temperatures are higher in a diesel engine, a 5W-30 oil would become much too thin to properly lubricate in those extreme temperatures.
The spread on cost differences between 5W-30 and 0W-40 can be quite dramatic. At the low end, a conventional grade 5W-30 can be as little as $3-4 per quart. A full synthetic 5W-30 can be between $8-12 per quart, depending on brand. A 0W-40, because it is only available in full synthetic and not as widely used, ranges between $12-17 per quart. Often it is worthwhile to pay for a full synthetic, because of the superior protection they offer.
Motor oil seems like a simple topic, but given the advancements in the modern engines these oils protect, with machined tolerances thinner than a human hair, oil has become incredibly complex. There’s always more to know, but with the basic knowledge in this article, you’ve got a good start.