As a result of consumer and government demands for ever-increasing fuel economy, automakers are increasingly replacing larger V8 and V6 engines with smaller, more fuel-efficient V6s and four-cylinders, many of them "boosted" by a turbocharger or supercharger to maintain power and performance. And, at the same time, more fuel-efficient turbocharged diesel engines are becoming more popular.

So do both types require special engine oil? In general, yes. As always, we at Kelley Blue Book strongly recommend using the grade and type of oil your vehicle's maker recommends, which is easily found in its owner's manual. But that being said, we believe that auto engines will respond well to oils designed for their special characters.

For starters, smaller engines have to work much harder than larger ones to provide the performance and capability you need and demand, and turbocharging or supercharging substantially increases their output. That means still-harder work and higher operating temperatures, the combination of which requires better, slicker, more heat-resistant oil.

"Turbochargers get very hot because exhaust gas turns the compressor turbines," said Quaker State technology director Jeff Hsu. "A lot of hot gas is coming through, and they spin at 25-30,000 rpm when fully spooled up, so a lot of heat is generated. A turbocharger also has little bearings with tight clearances on a common shaft where oil has to feed into them, so the oil can get cooked and a lot of coking can happen. Coking is sticky burned carbon that sticks on the surfaces and chokes off the flow like cholesterol in your blood vessels. You want to use an oil that's very high quality, probably in the synthetic realm like Quaker State Ultimate Durability Full Synthetic, to handle all that heat."

Diesels are a whole different animal burning different fuel through a different duty cycle, so they require a different kind of oil specifically formulated for the stresses they encounter. 

"Diesels need diesel oil," Hsu told us. "You're not going to use a gasoline engine oil in a diesel."

The diesel engine's oily fuel self-ignites from compression as the piston reaches the top of its travel inside the cylinder, instead of by an electric spark as in gasoline engines. That process begins with higher compression ratios and results in both higher temperatures and more combustion waste blowing by the piston rings into the oil below them. Much of that waste is soot, which consists of diamond-hard carbon particles that can make the oil dirty and gritty and lead to wear if not properly controlled. 

"Diesels make a lot of soot," Hsu explained. So do modern direct-injection gasoline engines, he added, though to a lesser degree, "because of the higher heat and the precision control of the fuel being delivered at the last minute. And carbon black in the oil is going to scour and cut surfaces."

The additional stress put on the engine - and its oil - by high-compression diesels make substantial demands. Auto manufacturers typically make diesel parts more robust than similar parts for gasoline engines, and it makes sense to use a more robust oil.

"Diesels are much higher-compression engines put together a lot tighter than gasoline engines," Hsu said. "They are stressed much higher; their energy efficiency is much higher; they're built with a lot more durability in mind, and they're all direct-injected and turbocharged for better performance. They might not be high in horsepower, but they are very high in torque, and torque results from a lot of pressure and stress in the cylinders. They are definitely a lot tougher with higher pressures that put a lot of stress on the oil, so they do require special oils."


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