Oxidation
"Heat is a catalyst for a chemical reaction between oxygen and oil, also known as oxidation," Mark explains. "When oxidation occurs, it attacks the chemical composition of the oil and changes its molecular structure, which forms acids and creates sludge. Due to the lower level of impurities in the cross-linked molecules of synthetic base oils, they are more impervious to the effects of oxidation. Combined with quality chemical antioxidant additives, synthetic oil can last for extended periods of time. If a mineral oil and a synthetic oil were formulated with the same additive package, the synthetic would last longer because it's not going to deplete its antioxidants as quickly."
Race Oil
From a lubrication standpoint, race oil is far superior to street oil. This is because street oil is bound by API restrictions, which limit the amount of antiwear additives that can be used in street cars, since it's believed that they can deteriorate catalytic converters when used in high concentrations. "All the good stuff you'd normally put in a racing oil you can't put into street cars because of limitations set by the API," explains Mark. "The government is more concerned with emissions than ultimate lubrication properties." That being the case, can racing oil be used in street cars? The PC answer: only if your manufacturer's warranty has expired; most racing oils have the detergents and dispersants necessary for daily use. In order to facilitate maximum performance, racing oils vary in viscosity compared to street oils, so be sure to consult with the oil manufacturer before making the switch.
Shearing
It's odd to think of a fluid as something that can shear, but that's exactly what happens to oil over time. In areas where oil is squeezed-such as in an oil pump, between rings and cylinder wall, and around crank journals-oil molecules permanently shear over time, resulting in a loss of viscosity. To compensate for viscosity loss as oil breaks down and heats up, viscosity index improvers are added to the formulation. "To put it simply, VI improvers have polymers that wad up under heat and behave more viscously," says Mark. "In the early days, the polymer chains in VI improvers would shear so much that a 5W-30 would turn into a 5W-20. However, with today's advances in modern polymer quality, that's no longer an issue. A big advantage of synthetic base oils is that they experience very little permanent viscosity loss and don't rely as much on VI improvers."
API
Think of the American Petroleum Institute (API) as the watchdog of the motor-oil industry. In conjunction with auto manufacturers, the API has developed a set of quality standards for various classes of oil. "Whereas the SAE tells you nothing about oil quality-just viscosity-the API actually tests the lubrication properties of an oil," explains Mark. New-car makers use the API to establish a benchmark for oil quality for warranty purposes on engines. "The API will routinely go around the country pulling random samples off the shelf to verify oil quality coincides with what a manufacturer claims, and also to make sure a package is labeled according to their standards," Mark says. "If something is wrong with an oil sample, the oil manufacturer is immediately notified to correct it."
Oil Grades
The API's quality standards are broken down into several classes, or grades, that rate oils based on their wear and oxidation properties. A two-letter designation-such as SJ, SL, and SM-is given to each oil formulation as an indicator of its lubrication properties. Every few years, the API establishes a new benchmark that supersedes all previous grades of oil. In theory, an SM-grade oil offers superior protection to that of an SL, and an SL formulation should outperform an SJ. However, this isn't always the case. "As API standards continue to evolve, these days it's more concerned with increasing oil life and reducing the potential for oil to contribute to emissions," says Mark. This has led many industry insiders to come out and say the newest SM grade oil falls short of the wear protection offered by the SL grade it replaced.