The jury's in, and it's official: Synthetic oil does a better job at engine protection than conventional mineral-based oil.
There are a bunch of reasons why this is the case, really. Conventional oil tends to thicken at low temperatures, making it harder for it to reach the upper end of the engine and valvetrain on cold days—and those first few seconds after startup are crucial for any engine. Conversely, conventional oil thins out at extreme heat, compromising its engine protection in summer. Synthetic, on the other hand, is much more stable at the molecular level and retains the same viscosity and pour properties across a wide range of temperatures.
The advantages don't end there. Synthetic oil is better when it comes to shear strength, which refers to the point at which molecules can be torn apart at extreme pressures. In addition, it's "greener," since it's produced in a lab rather than from fossil fuels that are extracted from the ground. Synthetic oil is also less prone to sludge buildup on crucial internal parts and can go longer between oil change intervals, which helps ease the sting of its higher price.
New-car warranties now mandate synthetic oil. But why?
- Thanks to improvements in design, assembly, and manufacturing, newer engines are built to closer tolerances than in the '90s. As a result, they tend to run hotter and require better protection, and synthetic oil cuts friction to keep those operating temperatures down.
- Rod bearings and crank bearings rely on a thin film of oil between themselves and their contact surface, and newer engines are designed with special polymer coatings on these bearings to cut friction and wear. These polymers are formulated to work specifically with synthetic oil rather than conventional oil.
- Turbochargers are driven by exhaust gases that are directed back into the turbo, so they run at extremely hot temperatures and high RPMs. Same thing applies for superchargers, although a supercharger is driven by the engine itself and not by exhaust gases. These accessories need the best engine protection possible, and that would mean synthetic oil.
- At extreme temperatures, conventional oils can "boil off" their additives, breaking the oil down and making it less effective at lubrication. Synthetic oils do not have this problem.
In other words...
Today's engines are specifically designed to run best on synthetic oil, and most call for low-viscosity oil to help cut down on drag and squeeze more efficiency out of the engine.
It's not just for new cars, though. At one time, rumor had it that you couldn't switch to synthetic in an older engine without developing leaks around gaskets and seals, or that you couldn't change back to conventional oil again if you did. That's all rubbish—you can use synthetic if you want, and switch back and forth all you want.
Best of all, like all emerging technologies, synthetic oil has dropped considerably in cost in the last couple of years. Maybe now the better question is: Why wouldn't you want to run on synthetic oil in your engine?