Can I mix oils in my Jabiru engine? And what about fuel?

James Missler of Bellevue, Ohio, is in the process of breaking in a Jabiru 2200 engine in his Kitfox. But he has a problem.


James Missler of Bellevue, Ohio, is in the process of breaking in a Jabiru 2200 engine in his Kitfox. But he has a problem.

“”For initial break-in at average temperature right now I should use an 80 grade mineral oil,”" he said. “”My local FBO only had 65 grade and 100 grade mineral oil. I purchased some of both. Can I mix the two to obtain approximately 80 grade or will the 100 grade plug my oil cooler at lower temperatures?”"

He has another question: “”The fuel requirement is 95 RON. What is RON? Are the octane numbers different in Australia where the engine comes from?”"

As to the first question, yes, you can mix a 65 grade oil and a 100 grade oil and come up with basically an 80 grade. The general rule of thumb in lubricating oil is that one can blend one grade above and one grade below the required viscosity grade, and the ending viscosity will be proportional to the percentage of each grade. That means a roughly 50/50 blend would result in an 80 grade oil. (I know that a 50/50 blend of a 65 grade and a 100 grade will give you an 82.5 grade, but that would be within the spec for an 80 grade.)

RON stands for Research Octane Number. For auto fuels, there is the research octane number, the motor octane number and the R+M/2 number. The R+M/2 number is just an average of the research and motor octane numbers. This is the number that is posted on the pumps at your local service station.

Octane numbers are a measure of a fuel’s resistance to knocking. The higher the number, the less the possibility for knocking to occur in your engine.

The research and motor numbers are both determined in a single cylinder knock test engine. In these engines, the compression ratio can be increased while the engine is running. During the test, the compression ratio is increased until knocking occurs. The compression ratio can then be correlated back to a known octane requirement.

The main differences between the research and motor octane numbers are the rpm and intake air temperature at which the engine is run during the test. As the motor octane number test is run at a high rpm and intake air temperature, this rating is usually about eight to 10 numbers less than the research octane number. Therefore, if you purchase a fuel with a posted R+M/2 of 90, the RON should be 94 or 95.

The test methods for determining octane number are set by the American Society for Testing and Materials (ASTM). These procedures are used in almost every country around the world, which means an Australian requirement of 95 RON will be the same as a 95 RON in the United States.

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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