Recently I received an email from a pilot who watched a video on YouTube comparing the performance of several different oils, including ones for aviation, automobiles, diesel engines, motorcycles, and boats.
The aviation oil included in the many different tests was Phillips 66 X/C 20W-50 Multiviscosity Aircraft Engine Oil.
The conclusion of the YouTuber’s testing was that the Phillips aviation oil performed very poorly in the detergency and anti-wear tests. The pilot who sent the email asked if there was something wrong with the oil.
This reminded me of a quote from Alexander Pope: “A little learning is a dangerous thing.”
This quote was used many times by one of my high school teachers and is very applicable to what I see on the Internet today. Of course, there is a lot of very useful information on the Internet. However, there is also a lot of incorrect and false information on the Internet.
The problem with the YouTube video is it compared automotive and heavy-duty engine oils to a piston aircraft engine oil that has a very different specification.
Although many people refer to oils like 20W-50 as a detergent oil, they are actually ashless dispersant oils. Military and SAE specifications forbid the oils from containing an ash-type detergent.
The same is true of zinc type anti-wear additives, which are death to any silver bearings or copper alloy exhaust valve guides used in some aircraft engines.
So what we have here is someone making an instructional video that has very little learning and, in fact, is very dangerous to pilots who tend to believe anything they see on the Internet.
For example, if a pilot switched from the 20W-50 to a heavy-duty engine oil because the video implies the heavy-duty engine oil is “better,” it would probably lead to pre-ignition, stuck valves, or bearing failures.
Multigrade vs. Single Grade Oils
Meanwhile, I keep getting questions on why I recommend multigrade oils in the winter and single grade oils in the summer.
This goes to how the viscosity of oils is determined.
To measure the viscosity of an oil, you start with a special glass U-shaped tube. In the bottom of the U is a bulge and on one of the legs is a small bulge and marks above and below that bulge. To run the test, you fill the bulge at the bottom of the U with the oil to be tested and place the U tube in a bath set at the needed temperature.
The tube sits in the bath for a time until the test oil reaches the prescribed temperature. Tests are usually run at 40°C and 100°C.
When the test oil reaches the prescribed temperature, a technician applies a vacuum to the side of the U tube with the bulge and draws oil up above the side bulge. Now they time how long it takes for the oil to flow past the mark above the side bulge down to the mark below the bulge.
This time used to be called Saybolt Universal Second, or SUS, but now is just converted to centistokes, which is a unit of measurement for kinematic viscosity, which is the ratio of a liquid’s dynamic viscosity to its density.
The important thing to note is that the high temperature viscosity is just a measurement of the rate that an oil flows under gravity with no pressure or shear force.
The low temperature measurement for the 20W part of the oil is measured in a cold cranking simulator.
To make a multigrade oil, companies can use a synthetic base oil or add a polymer, which is a long chain molecule that expands when heated and shrivels when cooled. For example, Aeroshell 15W-50 contains synthetic base oil as well as some polymer.
The problem with an oil containing polymer is that under high loads — like at the cam to lifter interface — the polymer can squeeze down and act like a lower grade oil.
During the cold winter weather, multigrade oils have a significant advantage in cold starting but, even more important, gets the oil to critical bearing surfaces quicker once the engine starts.
On really hot days, a single grade oil provides additional protection to critical high load bearing surfaces to that of a multigrade oil.
So by using a multigrade oil in the winter, you have better bearing protection when starting the engine. Switching to a single grade oil during the hottest part of the summer gives you the best protection during high-temperature cruise conditions.
I am not saying that multigrade oils are bad during the summer or that single grade oils are bad in the winter. What I am saying is that using a multigrade oil in the winter and single grade oil in the summer will give you the best protection for your engine and the best chance of reaching full TBO.
