Motor Oil
The myths, facts, and mysteries of the slippery stuff that keeps your engine happy.
By Gordon Jennings, Originally published in Motorcyclist, October 1996
Here's the bottom line when it comes to motor oils: you really cant go wrong by following the recommendations given in your owner's manual. Your motorcycle's maker has dyno-tested its engine with a crankcase full of the specified oil, or one with the same American Petroleum Institute (API) rating. You can be sure that particular oil will do the job. Will it do the job better than any other oil? Will it do the job if the engine is no longer as its manufacturer made it? Not Necessarily, as you will learn by reading further.
The API's ratings once went from 'SA' (guaranteed to be oil) through the alphabet to 'SE' and was extended to 'SE/CC' (you cant drive a nail through a film of this stuff.) Today the API has fewer performance grades, only 'SH' and 'SG' for spark-ignition engines, with an 'SH/CD' rating for oils good enough to be used in passenger-car diesels. The motor oils bearing these API markings have been test-certified for today's engines, which are in turn constructed with these oils in mind.
You should be aware that motor oils are now being compounded not just for lubrication, but to improve fuel economy as well. Oils have always been compounded with a thought for fluid drag; this is the first time its been made a priority. The API has two economy ratings: "energy conserving" for motor oils that yield a 1.5% reduction in fuel consumption as compared with a reference oil; "energy conserving II" is an oil that provides a 2.7% drop in fuel consumption.
As you might expect, energy conserving oils drag reduction benefits also show up as increases horsepower. Both thermal and mechanical losses diminish the power liberated in the combustion process on its way to the output shaft. The work of pumping air in and out of the engine accounts for the majority of the mechanical losses. The rest is mostly lost to fluid drag on the piston, which is, all other things being equal, largely a function of oil viscosity.
Friction exists even in the absence of actual contact between opposed surfaces. The cylinder wall's oil film normally prevents it from being touched by the piston, it is a source of friction itself, if we take that to mean a resistance to relative motion. Millions of molecules on each side of the gap try to stick together and get pulled apart. The sum of millions of molecule's minute resistance to separation comprises viscous drag, the source of most friction in a running engine.
Viscosity aside, the most important property of an oil is that it be "oily." Introducing any liquid between a piston and cylinder wall, for example, will reduce friction between the two surfaces. The degree to which friction will be reduced is, broadly speaking, a function of the liquid's viscosity. But maple syrup and motor oil of essentially identical viscosity do not lubricate equally, as you discover by rubbing samples of each between thumb and forefinger.
If "oiliness" were the only quality to be considered in choosing motor oils, we'd be squeezing all ours from castor beans. Castor oil, the smell of which once perfumed the air at motor races, is the oiliest of oils and it remains in some respects the supreme lubricant. It does oxidize too readily, however, forming ring-sticking gums and varnishes, and daubing fouling deposits on spark plugs. In a running engine, castor oil goes right to work gluing piston rings in their grooves and slathering gum and varnish everywhere. You wouldn't want it in any engine that can survive without its help.
But castor oil, a mixture of ricinoleic and triricinoleic glycerides, plus 10-12% of other fatty acids, remains one of the best lubricants for 2-stroke racing engines. Castor oil clings to metal with such tenacity it cannot be removed except by machining. It is an exceptionally effective film lubricant.
Oil forming a hydrodynamic wedge between surfaces keeps pistons and bushing-type bearings from metal-to-metal contact. Viscosity pulls the oil between a moving piston and its adjacent cylinder wall, or a shaft and a bearing, and pressurizes the gap. The pressure increases with viscosity and speed, and a well-designed engine almost totally prevents scrubbing contact.
The qualifying "almost" is needed because hydrodynamic action is not present in an engine at start-up, and it collapses around the pistons and rings at the end of their strokes. Under these conditions, parts are protected only by film lubrication, which is provided by the dipolarity of the oil molecules. The molecules behave like tiny magnets and adhere to ferrous metals and each other.
One of the great improvements in motor oils came circa 1950, when when the detergent/dispersant additives developed for diesels came into more general use. Alas these brought with them unfortunate consequences for old, high-milage engines. In those, the detergents sometimes dislodged great clots of oxidized oil filth to clog filters and oil passages. Engine failures caused by detritus liberated in this manner put additive oils in bad repute, with the results that some people still buy and use straight non-additive oils.
The first oil additive was probably the spoonful of sulfur old-time truck operators tossed into axle and transmission housings. The sulfur reacted with gear-tooth steel to give the gears an iron sulfide film. The film was important because the relative speeds between meshing gears is too low to form a fluid wedge strong enough to resist the extremely high gear-tooth loads.
Engines also have points at which loadings can exceed the carrying capacity of the fluid wedge. Take the tiny contact area between the exhaust cam and follower, for example. The load there rises to roughly 1500 pounds for every ounce of valvetrain weight at high engine speeds. Full throttle adds 80 pounds of load for every square inch of valve head area, meaning the load focused on the cam/follower can reach pressures in the order of 20,000 pounds per square inch.
Cams would have to spin much faster than they do (half crank speed) to work up a fluid wedge capable of carrying such high loadings. So the job has to be done with film lubrication, which means a more viscous oil, one with special properties (castor) or an extreme pressure (EP) additive. It's obvious that film lubrication is important, where some of us go wrong is in leaping to the conclusion that those who compound motor oils have overlooked this very point.
Dealer's shelves usually have a selection of flasks filled with liquids I like to call "mouse milk." This stuff reduces the friction in your wallet enough to make money slip out of it, but may not do anything else. About the best you can hope from mouse milk is that it will either be more of the same additives already in good-quality motor oils, or at least not get in the way of the additives that can do something useful.
Film-condition additives usually are chemically and/or thermally reactive. The sulfur- and phosphorus-based compounds react with iron to form slippery iron sulfides, as previously noted, or wear-resistant iron phosphides. Fatty acids, like those in castor oils, react with iron to make low-friction iron soaps.
Thermally reactive "liquid metals" like molybdenum dithiophospate, are oil-soluble chemical compounds; molybdenum sulfide, on the other hand, is a cheap dry-slide lubricant sometimes used in greases. If you put MOS or other dry-slide lubricant powders like colloidal graphite in motor oil, these solids may settle or filter out. Worse, they may become a barrier blocking the more effective reactive additives.
The liquid metals dissolve in oil, like salt in water, and remain in solution at all normal engine operating temperatures. But when friction heats the liquid metal compounds they come apart and their metallic component is plated on the hot spot. This stops the most potent, least obvious wear process in today's filter-protected engines: direct, scrubbing contact between a cam and follower, gear teeth, etc. This contact results in wear largely due to friction welding: Friction melts pinpoint areas of metal on both sides of the contact area, and they weld themselves together. These minute welded particles then break away, and after enough of them are carried off by the oil, the parts need replacement.
The role of liquid metals--usually molybdenum, tungsten, or zinc compounds--is both to interfere with friction welding, and to sacrifice itself to the wear that would otherwise devour engine parts. Unfortunately, phosphorous compounds degrade catalytic converter performance, so the feds limit the amount of additives like zinc dialkyldithiophosphate in motor oils. But in nearly all instances there is enough to last from one oil change to the next.
In the years before we had effective micron-level air and oil filtering, abrasive engine wear was a problem. The typical spark-ignition engine sucks in 10,000 gallons of air for every gallon of fuel it consumes. If you dont filter that air, it carries grit into your bike's engine post-haste. The larger particles do little damage unless they get caught between a valve and its seat, pitting both severely.
Virtually all dust particles are silica or silicon oxide, an extraordinarily hard substance with plenty of sharp edges. Engines with inadequate (or non-existent) air filters eat a huge amount of this grit. The good news is, most of it leaves with the exhaust gases. The bad news? What does stay can do severe damage, whether its in the wrong place or carried around in the wrong oil.
Modern air and oil filters trap just about everything larger than a micron (1/1,000,000th of a meter, or 0.000039 inch) in diameter. Particles of that size are enveloped by the oil film separating an engine's moving parts. Even a very light oil provides this protection. SAE 5 seems watery, but it has a film depth of not less than 0.001 inch, deep enough to submerge particles smaller than 26 microns.
Abrasive wear was a bigger problem back when the typical motorcycle air filter was a coarse screen capable of stopping nothing much smaller than pea gravel. The old gravel strainers gave free passage to the 20-micron grit that does the worst damage, especially to piston rings.
Fine grit was/is still a great killer of roller cranks. Grit becomes embedded in bearing cages and makes them depressingly effective crankpin grinders. You can prevent this kind of damage by using the thick oils, SAE 30 and above, envisioned and reccommended by the people who built these old engines.
Thick, high-viscosity oils are good for enveloping grit. They also do a great job of sealing and cushioning, which are two important functions of all motor oils (much more 30 years ago than today). The aluminum piston alloys in use circa 1960 had high expansion rates and poor high temperature strength. Accordingly, they needed to be surrounded by thick oil, to seal the fire trying to blow past the generous clearances--and to keep them from rattling in their bores.
Thick oils spread the concentrated loads between roller bearings and their races. The mechanism of rolling-element bearing failure usually is "brinelling," fatigue-related flaking, of the inner bearing race. Under load, the race under the roller (or ball) yields minutely as the bearing turns, just as a paved street yields to the weight of a passing truck. And in time the bearing race, much like the street, begins to break up.
Plain insert-type bearings can also fall victim to fatigue failure. You can bring about their early demise by feeding them a too-thick oil, which will turn into a too-thin oil in the bearing. The oil in plain bearings, whether connecting rod inserts or the floating bushings in a supercharger, is heated by fluid shearing. If the oil's viscosity and bearing clearance are properly matched, there will be sufficient oil flowing past the bearing to keep it cool.
When you pour SAE 40 into an engine designed for SAE 10-30, you may intend to protect its bearings with the thicker oil. But the increased oil viscosity , and resulting reduction of flow, can overheat the bearing. The metals used in plain bearings--copper, lead, and aluminum--typically lose half their ambient temperature strength at 200 degrees F. Copper-lead bearings are stressed near their elastic limit at redline crank speeds, even with crankcase oil temperatures below 250 degrees F. Pour in some thick oil, or a "mouse milk" viscosity index improver, and you'll reduce the bearing's oil flow, which will make it hotter and may cause it to fail.
Engine oils are viscosity-rated by subjecting them to the arcane arts of viscometry at 40 degrees F, then heating them and repeating the test at 210 degrees F. When you see a 10W30 rating on an oil, the "W" means the oil's base stock has actually been tested down to zero degrees F with a cold cranking simulator. It is assumed, for purposes of viscosity, that motor oils are "Newtonian" in that their loss of viscosity with temperature (meaning that their rate of loss is fairly constant.) The rate of loss is given as a viscosity index number, and in this respect some oils are better than others.
Multi-grade oils are made so by chemical additives called "viscosity index improvers." These additives contain either colloidally dispersed long-chain molecules that dissolve into true solution as temperature rides, or spiral molecules that open up and get longer with increases in temperature. Both of these actions "thicken" heat-thinned oil. Add the right VI improver and you get, for example, an oil that tests like SAE 10 at 40F, but looks more like SAE 30 at 210F. A multi-grade oil doesnt thicken with increased temperature, it doesnt thin as much as a single-grade oil.
One thing you should know about multi-grade oils is that their VI-improving additives will wear out. You can fool mother nature, but not forever: Long-chain molecules shear apart, so the the 10-30 oil you poured into your motorcycle's engine becomes 10-25 oil after a time, then 10-20, 10-25, right down to 10-10 if you cover enough miles between oil changes.
Over the last couple of decades we have seen the rise of "synthetic" and "synthetic blend" base stocks in motor oils. The big difference between plain old refined oil and synthetic is the the latter is, well, synthesized. When crude oil is refined, it is effectively sifted. The SAE 30 base stock you get in the sifting operation represents an average of molecule sizes, some being larger and others smaller. Shearing in a running engine breaks the big molecules apart faster than the little ones, which reduces the average size of molecules in the oil and thins it.
In contrast, synthetic base stocks' molecules are uniform in size, having been assembled out of fragments in a molecular stew. Synthetic oils also contain none of the waxes that can block low-temperature flow, and none of the instant-sludge crude-oil cruds or aromatics that vaporize and drift away the first time a spark plug fires anywhere near them.
I was not impressed by some of the early synthetic motor oils, which were compounded using cheap glycols as a base. Union Carbide's polyalkylenes oozed past gaskets and seals, some others synthesized from gases returned to gaseous form in the hot engine environment.
The better synthetic base stocks in use today are record-holders on the viscosity index scale. They still need a good squirt of VI booster to qualify as multi-grade oils, but they need less of it than refined base stocks. This is important, as polymeric viscosity index improvers' long molecules are unstable in shear. The less help your SAE 10W30 motor oil needs to meet its high temperature obligations, the longer it will be effective.
Good synthetic motor oils also have better non-newtonian, "apparent viscosity" behaviors. Oil displaying these "kinematically diminished" properties behaves like a thinner oil when rubbing speeds are high enough to build a thick fluid wedge.
Which synthetic oils are best in terms of apparent viscosity? I dont know, and neither does anyone who lacks a laboratory full of expensive, complicated equipment. I also dont know which additives, or how much of each, is present in the containers of motor oil--refined or synthetic--you'll see displayed at dealerships, service stations, and the like. That information is a closely held trade secret.
So after all this talk of motor oils, how do you tell good from bad? The bottom line here is, you buy the label on the container; you buy reputation. When you see a plastic bottle labeled "zowie lube," with small print that says it was packaged by "O'grady's Motor and Hemmroid products," put it back on the shelf and reach for something familiar. When I tell you to buy name-brand products, I'm not just sucking up to this magazine's advertisers.
Castrol is not an advertiser, but I will tell you the company has been making motor oils since we've had motors and I dont think it would knowingly sell you anything that would tarnish it's good name. I've used Castrol's motor oils for both racing and street applications, without disappointment. Refined-based GTX, sold super-cheap at supermarkets everywhere, is a very good motor oil and may be better than some higher-priced synthetics.
Mobil, which is an advertiser, long ago began developing synthetic motor oils and put its considerable technical resources to work creating a good one. They came up with Mobil 1, an oil using mostly polyalphaolefin base stock reinforced with a big percentage of polyol ester, the latter being an especially good lubricant in its own right. Mobil 1 probably is today's best widely-available motor oil. As a result of prepatory research I have done prior to writing this article, I bought (yes, bought!) Mobil 1 for use in my own vehicles
Red Line, an advertiser, is making a name for itself as a source of all-synthetic motor oils, and this company, like Mobil, relies on big percentages of polyol esters in its base stocks. My contacts in two- and four-wheeled racing tell me Red Line's oils are producing excellent results in everything from NASCAR's stockers to motorcycle GP racing's shrieking 2-stroke engines.
Keep in mind that your motorcycle was extensively tested with its cavities full of the lubricants specified by its maker. Motorcycle manufacturers dont test their models on oil specially compounded to keep engines, clutches, and transmissions happy, they instead do the sensible thing and design hardware compatible with the oils they know you'll be able to find. Its the smart thing to do, and it works right up to the point where you ignore their advice.
) I read his little "spoofs" each month and ALWAYS... seemed to finish his column... thinking to myself... "what in the hell... did he just say?!" Even wrote in... to the magazine once... for the Readers Column asking for a little more "lay-man's terminology" about an article this guy did on suspension, because after reading his column, that month... I was left... once again... thinking... "what in the hell..."! 
