If you could harness the energy found on Internet forums regarding oil choices, you could light a small city for years. The debate might be complex, but oil's main job is pretty simple: Keep moving metal parts from contacting each other. In addition, oil holds dirt, metal particles, and other contaminants in suspension so they can be carried to and caught in the oil filter. Oil also acts as coolant, carrying heat away from places that coolant passages don't reach, as well as preventing oxidation and corrosion of internal parts, and acting as a detergent to keep the piston rings and valves free of gunk leftover from combustion.
At a microscopic level, even seemingly smooth metal parts have rough spots called asperities. Oil has to be viscous (thick) enough to keep the asperities apart in an operating engine. The hotter oil gets, the more viscosity it loses—it gets thinner—so viscosity improvers are added to make it more viscous at the engine's normal operating temperature. Viscosity improvers (VIs) are molecules that act like springs. When they're cold, they're compressed and small. At higher temperatures, they expand and make the oil film thicker—but only up to a point. Get the VIs too hot, or run them so long that they all get sheared into tiny bits by moving parts like transmission gears, and the oil loses its ability to maintain viscosity when your engine needs it the most. The VIs in good oil are extremely durable, but they're expensive. Switching to less effective and less expensive VIs is one way an oil producer can create a more affordable end product. There's no magic in it, really.
Just about every good motorcycle oil does all of these things and contains all of these additives in varying quantities and qualities. Where some oils differ substantially, though, is in the base stock, the stuff into which all the additives are blended. Mineral oil, sometimes called petroleum or dinosaur oil, starts out as crude oil pumped out of the ground. It's refined to remove contaminants, but no matter how many times it's refined, what's left is still composed of various different molecular configurations, some better suited to be used for motor oil than others. Carp about the market price of a barrel of crude all you want, but it's still relatively cheap and easy to get, so mineral-based oil puts a smaller dent in your oil-change budget.
Unlike mineral oil, which comes out of a hole in the ground, synthetic oil is manufactured under controlled conditions, so its molecules are much more uniform. This gives it a number of advantages over mineral oil. It has better oxidation resistance, which lets you stretch the oil-change interval if you desire. Synthetic oil has a lower pour point so it flows better at low temperatures and circulates around a cold engine faster on start-up. The molecular consistency gives synthetics greater film strength at high temperatures, which is a benefit to highly stressed, hot-running engines. Synthetic oils also tend to maintain their viscosity over a wide temperature range and resist thinning at high temperatures and thickening at low temps.
Semi-synthetic oils split the difference between mineral and synthetic by blending the two in a single formula consisting of no more than 30 percent synthetic. You get some of the advantages of a synthetic oil, such as better oxidation protection and lubricity, at a price closer to that of a mineral oil.
Oh, and forget what you might have heard about mixing oils. If you're running mineral oil now you can switch to semi-synthetic or full synthetic simply by draining out the old oil and pouring in the new. What little mineral oil that remains in the engine will mix with the new oil and eventually get flushed out in subsequent oil changes.
There's some debate among scientists over the exact origin of "dinosaur oil," but they all agree it wasn't dinosaurs. Oil is commonly thought to be the decomposed remains of single-celled organisms that lived in lakes and oceans long before dinosaurs arrived on the scene. So forget cloning T-Rex to make more crude. Fill your bathtub with plankton instead, and check back in a few million years.
Little Bits of JASO Swimming in an API Broth
Motor oil is rated by various independent agencies under agreed-upon standards developed with the manufacturers. For cars and trucks, the big dog here is the American Petroleum Institute (api.org), which maintains the API rating system. But you might see another one: JASO. That's the Japanese Automobile Standards Organization, which has developed more specific standards for motorcycles and their smaller engines.
Look in your bike's manual and you'll see a recommended oil. Some manufacturers specify a particular oil—usually a "house" brand—but what you're looking for is a recommended API or JASO spec.
API's rating system for gasoline engines starts with SA, which denotes an oil without any additives, up to the current top spec of SN. Even if the manual for your bike specifies one of the older specs, you can use the latest, which is backward compatible with earlier formulations. But don't go lower than what the manual calls for.
The JASO spec indicates how the oil performs in a friction test on a wet clutch. Unlike cars, most motorcycles use the same oil to lubricate the engine, transmission, and clutch. Some automotive oils have so much friction modifier in them that the plates will slip instead of hooking up. Motorcycle-specific oil is labeled JASO MA or MA2, indicating that it's safe to use in a wet-clutch engine. One way to tell if an oil with just an API rating is suitable for a motorcycle is to look for the absence of an "Energy Conserving" or "Resource Conserving" designation in the API service "donut" on the bottle. In order to be called EC or RC, an oil needs enough friction modifiers to be trouble for wet clutches.
A word about weights, then. A designation like 10W-40 on a multigrade oil means it flows like a 10-weight oil at cold temperatures and like a 40-weight when it's hot. (The "W" stands for "winter"—not "weight.") A range of multigrades can be specified for the same bike depending on the temperatures you ride in, with several grades overlapping.
Confused? Don't sweat it. There's a way to cut through the market-speak—just read the manual. Use the specified oil or a better one in the case of bikes calling for a now-obsolete API rating—remembering the caution about EC/RC oils. That's the standard the engine is designed for and its warranty predicated upon.