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"Despite all the myths, synthetic motor oil mixes readily with mineral oil so there is no need to flush the engine prior to treatment. Used synthetic fluids can be safely mixed with mineral oils for disposal or recycling."
"Synthetic automatic transmission fluid can also be used in many manual transmissions for faster gear changes without increased wear. The possible exception is a worn transmission with bad synchros. The thinner fluid isn’t always able to slow the synchros fast enough to prevent gear clash. Non-synthetic ATF is in the same boat."
"Stick-shifted rearends endure the most extreme punishment. The enhanced film strength of synthetic gear lubricant boosts its load carrying capability to protect the ring gear from a nasty bite from the pinion when the clutch is sidestepped at six grand."
"It’s pretty amazing when you think about it—all those metal parts spinning away furiously but never making contact. Oil makes it all possible. In addition to serving as a buffer against wear, oil also must cool critical engine parts, pump easily to critical areas at low temperatures, remain stable at high temperatures, and keep internal components clean and free from varnish and corrosive deposits. It’s a tall order, and for more than a century petroleum-based mineral oil has been the literal grease between the wheels. But in the last 30 years there has been a steady growth in the use of synthetic oils.
Where Mineral Oil Comes From
Deep within the earth’s crust are vast reserves of petroleum crude oil. Over millions of years, the decomposition of plant and animal matter form pressurized pockets of liquid that literally burst to the surface when tapped. Over time, the flow diminishes and must be forced by pumping water beneath the crude to make it accessible. As found, crude oil is far from useful in automotive applications. It contains many impurities that must be removed through a distillation process that separates the crude into gases, fuel liquids, lubricant fractions, and heavier components such as asphalt. Further processing of the lubricant fractions removes many impurities such as phosphorus, sulfur, and metals.
The objective of the refining process is to isolate the desired base oils, also known as mineral oils. The problem is that after conventional refining operations are performed, a wide variety of chemical components remain that can affect the size and structural arrangement of the molecules. As a result, there may be weak links that break down and degrade the ability of the oil film to perform all of the critical tasks within an engine when operating conditions run to extremes. It is true that most commercially available petroleum motor oils are produced to a very high standard of purity, but the fact remains that some unknown/unwanted content is still present unless cost-prohibitive extra steps are taken during the refining process.
Because modern production engines are built with closer tolerances and higher operating speeds than ever before and are making more average power per cubic inch, petroleum-based oils have reached a plateau. Now consider the hot rodder and the unparalleled output of our stroked, nitroused, and roller cammed motors, and the need for maximum oil protection is perhaps greater than ever.
What About Synthetic Oil?
Synthesized in chemical plants by reacting components to make a product with the desired properties, synthetic fluids can be virtually anything the chemist needs them to be. Poly-Alpha-Olefins (PAO) are the most widely used synthetic industrial lubricants available today. They are similar to prohibitively expensive super pure parafinic mineral oil but contain no sulfur, no phosphorus, and no metals. And PAOs consist of identical molecules of pure hydrocarbons that can withstand high temperatures without decomposing. Having eliminated mineral oil’s greatest weakness—unwanted molecular “hitchhikers”—the consistent molecular structure of synthetic oil is clearly superior.
So why isn’t synthetic oil in every engine, transmission, and differential? Because it costs more to produce. The key ingredients are decene molecules. Decene is a linear molecule with 10 carbons, and it’s synthesized by first linking together five molecules of ethylene, each of which contains two carbons. The second synthesis step involves polymerization of the decene. Two or more decene molecules are combined to form short chain-length polymers, and from these, PAOs result. No doubt, it’s a capital-intensive manufacturing process that unavoidably leads to higher retail prices than cheaper-to-produce mineral oil.
Additives
So far we have looked at mineral and synthetic base stock. But that’s only half the story. Chemical additives must be introduced to impart new or enhance existing performance characteristics of the base oil to give the resulting lubricant the needed properties to do its job. The ratio of base stock to additives ranges from 75/25 to 85/15 with base stock accounting for the greater volume. Typical additive agents include detergents to reduce the formation of residue, seal conditioners to prevent harm to rubber and synthetic seals while helping to keep them flexible, defoamants to deter the absorption of air, anti-wear agents, friction modifiers, dispersants, and antioxidants.
Viscosity is determined in large part by the presence of additives called viscosity index improvers. Motor oil changes viscosity as its temperature changes—it’s thicker when cold and thinner when hot. Ironically, it needs to act in almost the opposite way. At low temperatures, you’d prefer oil to be thinner so that it flows readily and won’t thicken too much or gel in extremely cold weather, reducing protection and making the engine hard to start. Yet at high temperatures the oil must be thick enough to maintain a critical film to prevent metal-to-metal contact. The ideal oil viscosity must strike a balance between low temperature flow and high temperature protection. Multiviscosity oil is formulated so it can safely be used over a wider temperature range than single-grade oil.
Thanks to additives, multiviscosity oil is possible, and in a quart of 10W-30 for example, you have an oil that acts like 10-weight at cold temperatures and a 30-weight at normal operating temperatures. In this universally adopted rating system, a smaller viscosity number indicates a better ability to flow at lower temperatures, a higher viscosity rating number indicates a thicker, harder to displace film at higher temperatures. Without the proper additives, this seeming twist of logic would not be possible.
Let’s Test
To see the difference between mineral-based and synthetic fluids, we enlisted the help of Scott Crouse’s ’65 Mercury Comet. With a hot 347, World Class T5 manual transmission, and 4.11 geared 9-inch rear axle, it’s a rolling torture chamber for vital fluids. First we made a series of runs on the Westech Performance Superflow chassis dyno with 20W50 in the crankcase, Dexron III in the gearbox, and 75W90 gear oil in the TracLoc differential; the result was 408.3 horsepower and 405.1 lb-ft of torque. Then we drained the petro-chemicals and replaced them with man-made hydrocarbons from Royal Purple: 7 quarts of 20W50 synthetic engine oil, 6 quarts of Max ATF, and 2½ quarts of Max Gear 75W90. After a 5-mile jaunt to get everything up to the same temperature as the baseline test, we let it rip. The monitor read 418.4 hp and 411.2 lb-ft of torque, a gain of 10.1 hp and 6.1 lb-ft just by switching to synthetics: an impressive tribute to the reduced coefficient of friction. We’ve seen similar improvements on the engine dyno, and have noted reduced wear through the use of synthetics. They’re a bunch more expensive, but in our opinion, they’re worth it for cars you care about. For your $200 Pinto, stick with the 99-cent stuff."
