What’s In Motorcycle Engine Oils?

Only a bit more than three-quarters of it is actually oil

The largest constituent of engine oil is its base stock(s), made up of long-chain hydrocarbon molecules. The most basic fact about such oils is their viscosity, which is their internal friction. When parts begin to move, it is viscosity that causes oil to be dragged between parts, forming a layer that in hydrodynamic lubrication completely separates the parts such that zero wear takes place.

The condition for this full-film lubrication is that viscosity must drag oil into the loaded zone between parts at least as fast as the load squeezes oil out. Provided there is enough viscosity, and given that the relative parts motion is rapid enough, it makes no difference what kind of oil is separating the parts; when there is a full hydrodynamic oil film there is zero surface-to-surface contact and zero wear.

As we know, engine oils contain an array of additives serving various purposes. Here is a list of the elements in a typical engine oil and their approximate amounts:

  1. Base stock(s), 78 percent: This is the part that is "just oil," whether petroleum or synthetic.

  2. Viscosity index improver, 11 percent: This additive makes an oil "multigrade" by slowing the rate at which it loses viscosity as its temperature rises.

  1. Ashless dispersant, 6 percent: AD molecules surround particles of varnish or gum, preventing them from clumping together or sticking to parts surfaces, such that they are carried to the oil filter where they are removed.

  2. Metal detergent, 3 percent: Similar to above, often with ability to neutralize acids.

Present in small amounts:

  1. Inhibitor: anti-rust, anti-corrosion.

  2. Anti-wear, such as ZDDP: Forms a solid lubricant layer on points of contact, preventing scoring and other surface damage.

  3. Pour point depressant: At low temperature, prevents formation of wax-crystal networks that can solidify the oil.

  4. Friction modifiers: Form adsorbed layers of long-chain oil-like molecules on parts' surfaces to make them more "slippery."

  5. Anti-foams: Speed air release by reducing the surface tension of air bubbles.

In sum, modern oils are roughly 22 percent additives! To learn more, Google “Technical Committee of Petroleum Additive Manufacturers, Europe.”

I was particularly interested to read in that article about rapid heat-driven polymerization of oil (polymerization is the joining of molecular elements into larger chains). The reason for my interest was overhearing an angry conversation at Road Atlanta sometime in the 1970s. The aggrieved party had used a recommended oil in his very hot-running racing four-stroke only to find, “That [expletive] stuff turned right into Jell-O in my engine! I could hardly pull out the [expletive] dipstick!” Antioxidant additives exist to prevent this.

Another point of special interest for me was to learn that oil oxidation can form corrosive organic acids, which attack nonferrous surfaces (especially plain bearings). This was the “hen-tracking” occasionally seen on plain bearings not protected by a lead-indium overlayer.

For many years, aircraft engine oils could not contain detergents because their metal component could whisker spark plugs, causing a cylinder to cease firing. The result was the heavy dark sludge I found in the accessory drive cases of the three giant aircraft piston engines that once plugged up my little shop (I later sent them to good homes).

It was tempting to reach in and scoop up some of that beef gravy-looking sludge with my fingers, but I knew it was full of sharp bits of metal. Why? Because if one cylinder says “no” and the other 27 cylinders say “yes,” a lot of damage can be done. The coming of metal-free ashless dispersants made it possible to deal with the aircraft engine sludging problem.

Why would aircraft engines whisker plugs when this is almost unheard of in bike or car engines? Street-driven vehicles seldom operate at high throttle for more than a few seconds, so their combustion conditions don’t promote plug whiskering. Aircraft engines spend most of their lives around 50 percent throttle, producing higher combustion temperatures.

We would sometimes take racebike plugs with weird red or green colors on their ground wires to the late Bobby Strahlman (“the Champion man”) for inspection. He’d glance at them and say, “That’s an oil additive doing that.” A drop of oil is filled with complex, ingenious, and essential chemistry.