There are three major differences between conventional and synthetic oils. Synthetic oil is more stable against oxidation and breakdown, has a higher viscosity index, and demonstrates lower volatility.
Because of its high stability, synthetic oil protects against breakdown, which can lead to residue formation (called “coking” and “sludge”) and engine deposits, and results in reduced engine wear. The engine runs cleaner, and the oil has a longer life.
Perceived as thickness or resistance to flow, viscosity is a measure of a fluid’s resistance to deform under stress. Viscosity is oil’s most important characteristic, and the level of viscosity is specified based on engine design and operating temperature. If an oil’s viscosity is too high or too low, it will cause undue stress to the engine, breaking down the oil structure and wearing out the engine.
An oil’s nomenclature indicates its viscosity. In the reference to 5W-30, the 5 is a relative measure of viscosity at low engine temperatures, and 30 is the relative viscosity at high engine temperatures. The lower the numbers, the easier the flow rate.
Synthetic oils have a more stable viscosity over an engine’s operating range.
This is an oil’s tendency to lose its lighter components through evaporation. Volatility is a function of the base oil. Synthetic oils tend to have lower volatility, which helps to reduce emissions and residue as well as improve service life.
In 2010, Subaru of America, Inc. partnered with Idemitsu Lubricants America Corporation to prepare Subaru-branded synthetic motor oil for applications in the new FB four-cylinder engine and in turbocharged engines. Researching and formulating Subaru engine-specific 0W-20 and 5W-30 synthetic motor oil ensures quality, performance, and, ultimately, customer satisfaction. This oil is available to do-it-yourselfers at Subaru dealers.