Shopping for engine oil isn’t like shopping for oil at the grocery store. In the world of motor oils, there is no singular pure oil like vegetable oil that works fine in every engine. Motor oils are all complex blends of base oils and additives in ratios of as much as 70% base oils to 30% additives.
Most oils have several synthetic or natural blends of premium and lower grade oils to serve different functions. These oils may have molybdenum and graphite added as friction-modifiers. They likely contain antiwear agents made of zinc and phosphorous to protect metal surfaces when ordinary lubrication is not there. Some cheaper base oils are more volatile than others and will lead to thickening of the oil and sludge. The oils contain detergents, anti-corrosion inhibitors, foam inhibitors, pour-point depressants, and dispersants.
Most handymen have a general idea of what those numbers on a bottle of oil mean. Numbers such as 10W-30 and 15W-40 represent the viscosity rating of the motor oil. You may have read in your owner’s manual that the engine requires a 5W-30 in the winter and a 15W-40 in the summer. The viscosity is the oil’s resistance to flow. Water has a low viscosity. Honey has a very high viscosity or density that prevents rapid flow.
The “W” in the number signifies “Winter.” In truth, it represents the cold start viscosity rating of the engine oil. A thicker engine oil may not flow adequately enough in winter to all the parts in need of lubrication. Some synthetic oils are available in numbers as low as 0W for ultimate cold start protection in high-performance engines. The larger second number signifies the viscosity rating of the oil once the engine reaches operating temperatures. Even though the second number represents a thinner hot oil, the number is larger because the viscosity rating is relative to the temperature of a sliding scale.
A thicker weight oil will help seal the pistons better and protect the engine bearing surfaces. High-mileage engine oils typically have a higher viscosity than they are actually rated. They are just below the next grade of viscosity in many cases. This helps to seal worn piston rings and increase the overall flow of the mechanical parts bathed in this thick motor honey. Try a 15W-45 Shell Rotella oil in your aging engine during the summer months and you will notice quite a bit of improvement.
Having a high viscosity index (a resistance to thinning) is only part of the equation. Your oil needs to be made of a durable base or have a combination of additives that work effectively with your vehicle or application. Depending on the temperatures and designs of the engines, some oils may last longer than others. An engine that is forced to endure more extreme temperatures and combustion pressures likely requires a special additive package to extend engine life.
Oil is a little more complicated than you would think. Some of the oils are loaded with additives or cheap base oils that can do more harm than good. Next time you are shopping for motor oil, consider researching what you are buying a lot deeper.