Which is better? Multi-grade of single grade oil?

For the past 20 years, an older gentleman has sought me out at Oshkosh to ask me the same question: “”Which is better in an aviation piston engine: single grade (SG) or multi-grade (MG) oil?”” Every year I feel that he wants a quick one-or-the-other answer, but every year I gave him a long, qualified answer that depended on what he is flying and where he lives. The main difference between most sales people and most engineers is that the engineers have very few black and white answers. To an engineer, the answer to just about every question depends on a lot of variables.


For the past 20 years, an older gentleman has sought me out at Oshkosh to ask me the same question: “”Which is better in an aviation piston engine: single grade (SG) or multi-grade (MG) oil?”” Every year I feel that he wants a quick one-or-the-other answer, but every year I gave him a long, qualified answer that depended on what he is flying and where he lives. The main difference between most sales people and most engineers is that the engineers have very few black and white answers. To an engineer, the answer to just about every question depends on a lot of variables.

The number one advantage of MG oil is cold temperature starting. When you start an engine at, say, 20?F, MG oil will allow the engine to crank more easily and have a better chance of starting. Once the engine starts, the big advantage of MG oils is that you get oil flow to critical bearing and wear surfaces faster. For example, in one series of tests at 20?F, the MG oil got to all of the bearing surfaces in four seconds while grade 100 oil took 10 seconds. This is a significant difference, and it can result in a difference in engine wear over the full TBO cycle. The MG oils also eliminate oil cooler plugging in even the coldest weather. However, if you live in a warm climate and never experience cold starting or cruise conditions, then these are not big advantages.

There are other, smaller differences between SG and MG oils. One of the most noticeable is that MG oils leak better than SG oils so, if you have a leaking engine, you probably will notice an increase in oil consumption and leaks if you change from SG to MG oil. You can also see some loss of prop control on engines that have excessive clearance in the constant speed prop oil transfer collar or other parts of the prop control system. Conversely, MG oil usually reduces oil consumption past the rings. This means that, in most tight engines, you will see a reduction in oil consumption if you switch from an SG to an MG oil.

You may also experience a change in oil temperature. Many engines will run with a lower oil temperature when you switch from an SG to an MG, although I have seen engines that operate with higher oil temperatures with MG than with SG. This can be important if your engine is operating at an oil temperature of, say, 170?F. Since that is at the low end of the acceptable range, changing to an oil that lowers this significantly can cause an increase in water content and rusting in your engine.

The biggest misconception about MG oils is that SG oils coat parts better. That idea comes from people placing a metal part in a container of SG oil and observing the amount of oil on the part. When they do the same procedure with MG oil they notice less oil on the part and conclude that the MG oil does not coat as well as the SG oil. As the procedure usually is done at room temperature, it is just a demonstration of the physical characteristics of the two oils. A 20W/50 MG oil has the viscosity of a 20-weight oil at cold temperatures and the viscosity of 50-weight oil at 100?C. (MG oil does not get thicker as it is heated; it just thins out at a slower rate than an SG.) If the same test were run at 210?F, then you would observe a very similar coating of the parts by the SG and MG oils. You should also consider that the parts in your engine are usually at around 200?F when you shut it off.

MG oils give a small improvement in fuel consumption (about 3-4%), which can offset the increased cost of the MG over the SG oil. MG oils also can show an advantage for race and aerobatic engines that normally run, shall we say, a little above normal temperatures and rpm’s.

So what is the best answer? Many pilots like to use SG during the warm summer months and then switch over to MG in the winter. Others prefer using only one oil so, if they live in a cold climate, they use MG year around. If they live in a warm climate or in a cooler climate but always have the plane hangared and well pre-heated before start-up, SG oil may work for them year around.

And that is as close to a definite answer as I get.

Ben Visser is an aviation fuels and lubricants expert who spent 33 years with Shell Oil. He has been a private pilot since 1985. You can contact him at Visser@GeneralAviationNews.com

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