How Oil Earns Its Keep

Motor oil performs four basic functions; lubricating, cooling, cleaning and sealing. It lubricates by providing a thin film of oil that buffers rod and main bearings from the crankshaft journals. Further, motor oil's slipperiness also reduces friction.

Cool Hand Lube

Oil cools an engine's moving parts by absorbing friction- generated heat and carrying it away with the oil flow. It's interesting to note the water/antifreeze coolant system provides but sixty percent of the engine's cooling. The balance of the heat is carried away to the crankcase sump.

Besides carrying away heat, the flood of oil washes rod journals and valve lifters clean so abrasive particles don't scuff the relatively delicate surfaces. Here's where that detergent rating struts its stuff. The detergent disperses contaminated particles, and keeps them suspended in the oil. When the oil's changed, the particles drain out with the oil instead of building up in the sludge. Finally, oil seals the rings against the cylinder walls, preventing loss of horsepower and fuel economy.

Death Of An Oil

The fly in the ointment is that engine hours reduce the ability of motor oil to fulfill its duties. First, latent engine heats acts like the fire in the refinery, breaking the motor oil down into lighter and heavier molecules. The lightest of the molecules float off into the atmosphere contributing to air pollution. The heavier molecules also create problems by thickening and transforming into sludge, gum and varnish.

But because they are not mineral based, as are conventional motor oils, synthetic lubes resist this mini-refining process, and the resultant break down of the oil. And because synthetics are slipperier than petroleum based lubricants, less energy is lost to friction.

Engines Run Cooler

Less friction means less heat. Synthetically lubricated engines typically run from 5 to 20 degrees cooler than their counterparts. That results in less engine wear, more available horsepower and up to a ten percent improvement in fuel economy. Besides superior lubrication, the most prized virtue of synthetic oil is that oil changes can be extended from four to eight times the norm.

Too Good To Be True?

There are, however, certain disadvantages. First, is price. Synthetics cost from two to three times that of the conventional mineral oils. Additionally, synthetics shouldn't be used in an engine that burns oil. Obviously that's because you'd be burning up very expensive oil, but additionally because engines that burn petroleum based oil tend to burn even more synthetic. Moreover, if the engine already leaks precious conventional oil, it will leak even more synthetic.

Worst Possible Case For Boats and RVs?

Some say the complications are even worse for boats and RVs whose engines are infrequently run. Naysayers cite tests that have allegedly shown automotive synthetic oils don't provide adequate rust protection for mothballed boats and motorhomes. This fable is not true, but it's understandable that the story at least got started.

The first generation of synthetic oils were used for racing, and were based on glycol. They had virtually zero ability for preventing cylinder wall rusting. However, the second, and current generation of synthetics are exemplary at inhibiting rust. Additionally, marine motor oils are packed with additional additives for rust prevention.