During the early 1990s, Rob Muzzy, then running Kawasaki’s AMA Superbike team, began to receive for test lower-viscosity oils with surprising numbers on them like 0w-15. These thinner oils were being introduced because auto manufacturers were employing every possible means to meet the Corporate Average Fuel Economy (CAFE) numbers required by the federal government.

In a warmed-up and normally running auto or motorcycle engine, there is little or no metal-to-metal contact between moving parts. Pistons and their rings glide on thin but complete oil films and crankshaft main and con-rod bearings likewise; otherwise engines could never routinely run as long as they now do without grinding themselves up into wear particles. This being so, much of the friction loss in engines comes from the power necessary to overcome the fluid friction—that is, viscosity—of the engine oil that is supporting the loads.

The now wide use of such thinner oils was inspired by famous racing stock-car driver and builder Junior Johnson. At a point, he was asking himself why racing V-8s have so much oil flying around inside of them. Serious friction loss is created when the crankshaft bats such oil around, losing energy to it and raising oil temperature. Use of lower-viscosity oil was attractive, with smaller crank-bearing clearances, but there was a problem.

Honda Gold Wing
What does the owner's manual say? For the current six-cylinder Gold Wing, Honda recommends API service classification SG or higher 10w-30—4.6 quarts after draining the engine, 4.9 with filter replacement.Courtesy of Honda

Oil supports loads by forming a wedge into which parts motion sweeps oil, in the process generating very high pressure. Applied load from parts inertia and combustion forces pushes the crank journals off-center, thereby creating this wedge. But when heavy load pushes the journal off-center this way, the minimum clearance between journal and bearing surfaces can become as small as 1.5 microns, which is 0.00006 inch or 1/60th of the diameter of a human hair. If we now reduce the viscosity of the oil we are using, that 0.00006-inch minimum oil film thickness will become even thinner, maybe enough so that the tips of the surface roughness of the crankpin will begin to touch those of the bearing (all surfaces are rough on an atomic scale).

Okay, how do we make the journal surface smoother? A routine operation in the engine-rebuilding industry is to grind crank journals slightly undersized (0.010 inch) and then fit undersized bearings. Crank regrinders used to offer a process called micropolishing. It turned out this wasn’t the answer; instead of making the crankpin truly round and smooth, it just made the tiny hills and valleys in journal surfaces shiny—like polishing shoes.

Serious friction loss is created when the crankshaft bats such oil around, losing energy to it and raising oil temperature.

More research revealed that a process did exist for making crank journals accurately cylindrical and very smooth but it was no longer available in the US. The Chrysler Superfinish process had been developed for shortening the break-in of military aircraft engines during WWII but was considered an unnecessary expense for mass-produced auto engines. The process was, however, available from an outfit in Germany. Johnson went ahead with his research.

The upshot of all this work and testing was that lower-viscosity oils were made usable—first in racing engines and then in production stuff—and their use did significantly reduce friction loss. As CAFE standards have tightened, saving that loss became necessary for manufacturers, enough so that surface finishes on cylinder walls and crank journals have been improved to make the use of lower-viscosity oils and closer clearances practical.

I saw a direct result of this some years ago when I rented a car with just 7 miles on its odometer. I thought, “Let’s have a look at the oil now, and look at it again in four days when I return this car.” When I pulled the dipstick, I saw clear amber oil—lovely. Then I plunged into days of morning and evening Atlanta rush-hour traffic, moving at 85 mph.

If your engine doesn’t have the smoother surface finishes required to make those lower-viscosity oils carry the load, they won’t carry it.

As I was filling the gas tank before returning the car, I lifted the hood and pulled the dipstick a second time. The oil was still clear and amber-colored, despite my 236 miles of high-speed commuting. Why wasn’t it black from break-in? Because manufacturers had switched to extremely smooth surface finishes so that energy-saving lower-viscosity oils could safely be used. Those finishes were so smooth that old-time “black-oil” break-ins were no longer taking place.

In the old days of medium-grit 60-degree crosshatch-hone cylinder preparation, that oil would have been black with the wear particles that break-in had generated. The old crosshatch-hone pattern turned engine cylinders into a file that quickly shaved the piston rings into an intimate fit in the cylinders, and the oil would be black. Today, with prelapped piston rings and cylinders that are round, straight, and finely finished, there is very little bedding-in to be done.

After skimming articles about energy saving with thinner oils, a few people have gone out and switched their older cars or bikes to low-viscosity oils that didn’t exist when their vehicles were built. Liquid horsepower, man! Sorry, if your engine doesn’t have the smoother surface finishes required to make those lower-viscosity oils carry the load, they won’t carry it. You’ll have metal-to-metal contact and failure.

This is just one more reason to read the owner’s manual and use the oil viscosity and grade called out in it, the oil that your manufacturer has found able do the job.