Ask Paul: Why is my engine oil creamy?

Q: I am doing a 100-hour inspection on a Piper Aztec with TSIO 540 Lycomings using Shell 15/50 oil with 50 hours on the oil. They are both midtime engines on this photo plane. They fly six hours a day at low power settings. The left engine oil is black like it should be after 50 hours. The right engine oil is a light creamy color. Something is not right.

I think this indicates moisture in the oil. The pilot says it was like this at the last oil change. The oil temperature is the same on both engines. Whats going on here?

George Gould, A&P I-A@Scholes Field, Galveston, Texas

A: George, I agree with you. This creamy oil indicates moisture to me too.

Of course the typical operation of a photo plane certainly isn’t the best operating envelope for keeping an engine at “normal” operating temperatures. You know, we like to see engine oil temperatures run between 185° to 210°F during routine flight. My suspicion is if we’re not getting the oil temperature in that range, we may be seeing the results as creamy oil.

I’m curious as to when this creamy oil began to show up? With the temperatures you have in the Galveston area, I’d expect it would be no problem to get the engine oil temperatures in the range we’d like, especially during the summer months.

If this creamy oil situation has only come about since you’ve had colder ambient temperatures, then I’d suggest you check the actual oil temperature in the engine oil sump using a laboratory type glass bulb thermometer.

You can either fly the aircraft or complete a good ground run where engine operating temperatures are normal. Then remove the oil dipstick and slowly — using extreme caution — insert the thermometer down the oil dipstick tube until you feel it touch bottom, then carefully pull it back out about an inch or so. Give it a minute to get a reading and — now again with extreme caution — slowly pull the thermometer out and check the reading. You want to compare this with the cockpit gauge just to see how close they are.

If the temperature is actually lower than we’d like to see, then I’d suggest blocking off part of the oil coolers to see if we can get the oil temperature to come up within our target range.

Note: If you don’t use extreme caution during this troubleshooting procedure, you will then learn how to remove an engine oil sump so that you can remove the glass mercury from the broken thermometer! This is guaranteed to increase your vocabulary in a matter of seconds.


  1. C Gerker says:

    Ask the owner if the twin is equipped with an after market air oil separators. These are questionable culprits in the system that will cause water vapor to condense back into the recovered oil and end up in the sump again.

  2. Tim Fountain says:

    A better alternative is to use a long bore thermocouple or PRT measuring device. You’re asking for trouble with a mercury based thermometer, as you indicated! Even an Infrared thermometer reading down the oil fill tube should give a pretty good core temp reading.

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