I was reading an article in AVweb, an aviation publication titled "
The Ten Biggest Lies About Piston Aircraft Engines", when low and behold, there was an section about oil. This doesn't directly apply to motorcycle applications, but I thought it was interesting none the less. The whole article can be found at
http://www.avweb.com/news/maint/182849-1.html?redirected=1-RP
Lie #3:
Modern multi-viscosity oil offers superior lubrication and longer engine life than old-fashioned single-weight oil.
During the 70s and 80s, there was a dramatic shift from single-weight to multi-viscosity oils by operators of general aviation aircraft...due in large measure to very effective advertising campaigns by Shell and Mobil that touted their multi-vis products (Aeroshell 15W50 and Mobil AV 1) as the greatest aeronautical innovation since the nosewheel.
During the same 20-year period, there was a dramatic increase in premature engine problems in the owner-flown G.A. fleet. It was not a coincidence.
In contrast to "working airplanes" that fly almost every day, most owner-flown airplanes spend most of their lives in the chocks. The biggest enemy of their engines is not inadequate lubrication. It's rust.
Multi-vis oil simply does not provide as effective protection against rust as single-weight oil. The defining characteristic of multi-viscosity oil — the fact that it doesn't thicken up at cool temperatures — makes it a lousy corrosion inhibitor. During periods of disuse, multi-vis oil strips off cylinder walls and cam lobes much more readily than does thick single-weight oil, leaving those parts vulnerable to corrosion, followed by spalling and eventually destruction.
But what about the superior lubricating properties of multi-vis oil? Basically bunk!
It turns out that multi-vis oil is not a better lubricant than single-grade oil. It's actually a bit worse. The reason is that multi-vis oil is made by starting with a thin, single-weight oil stock and adding man-made polymers called "Viscosity Index improvers" that increase viscosity as temperature increases. However, such VI improvers are not lubricants, and their addition actually displaces a certain amount of lubricating base stock (on the order of 10%). In other words, there's more "oil" in a quart of single-weight oil than in a quart of multi-vis.
Now this is no big deal, since the lubrication demands of most piston aircraft engines are rather modest (compared to automobile engines, for example). What is a big deal is the fact that single-weight oil does a better job of protecting engines against rust during period of disuse. That's why we've long recommend single-weight oil for any engine that doesn't fly at least once a week.
Fortunately, after two decades of multi-vis mania, it now appears that more and more G.A. operators are starting to recognize the shortcomings of multi-vis oil and are switching back to single-weight. An increasing number of top-rated overhaul shops are now recommending the use of single-weight oil.
