Are multi-grade oils bad?

In my last column, “”Multi-grade vs. single-grade oils: The debate continues”" (Feb. 16 issue), I recommend single-grade engine oil over multi-grade oils for use in large radial engines (over 1,800 cubic inches displacement). I am hoping that you all did not read that and conclude that if multi-grades are not as good in large radials, they probably won’t work in a 152 either. This is not the case.


In my last column, “”Multi-grade vs. single-grade oils: The debate continues”" (Feb. 16 issue), I recommend single-grade engine oil over multi-grade oils for use in large radial engines (over 1,800 cubic inches displacement). I am hoping that you all did not read that and conclude that if multi-grades are not as good in large radials, they probably won’t work in a 152 either. This is not the case.

When radial engines were designed, the engineers were aware that the master rod bearing was a lubrication concern because of the constant load. A large number of different bearing materials was tried before they settled on a silver alloy material that seemed to stand up better than most. (The manufacturers of two-cycle railroad diesel engines also use a silver alloy wrist pin bearing because these bearings are never unloaded.)

Large radial engines were used quite successfully in airline service. One reason was that flight profiles were carefully controlled and the engines were flown by the book. This meant that the engines did not experience sharp power or rpm changes and no reverse loading. When the radial engines were used in fighter aircraft, they were not really concerned about reaching TBO. In addition, the aircraft had a crew of maintenance personnel who would change out the engine when metal or problems were first noted.

Now many of these great aircraft are being restored and flown at airshows. In an aerial performance, many aircraft experience sharp load and rpm changes ? and occasional reverse loading does occur. It appears that, under these severe conditions, single-grade oils offer an additional safety margin.

So why is a single-grade oil better than multi-grade oil if they meet the same specification? For example, a 15W-50 or 20W-50 will normally have the same viscosity at 100?C as a single grade SAE 50 or aviation grade 100 oil. Part of the problem is that viscosity is only a measurement of how fast oil will flow out of a small tube. There is no pressure other than gravity on the oil. Under the high pressure of a large engine master rod bearing, the multi-grade oils will be squeezed out of the bearing cavity faster than a single-grade oil of equal viscosity.

To complicate this even more, single-grade AD oils are actually kind of like a multi-grade oil. To blend a single-grade 120 Ashless Dispersant (AD) Mil-L-22851 oil, a refiner starts with a grade 100 Mil-L-6082 mineral oil and adds a dispersant viscosity index improver to increase the viscosity to that of a grade 120. So when these engines are operated on a 120 grade AD oil, they are already using oil that is thinner than the oil for which the engine was designed.

So are multi-grade oils bad? No, it is just that the improved flow characteristics of the multi-grade oils aggravate a problem in these large radial engines that were designed before I was born ? and I’m old. Sometimes improvements are not universally better in all applications.

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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