Piston engine oil temperature
 

Q. If the oil temperature gauge is showing an excessive oil temperature and you trust that the gauge is actually working, would you expect to see:
A. High oil pressure
B. Low oil pressure
C. No change in oil pressure
D. Excessive sweat from lone pilot

Answers not on a post card please.
Regards,
RAPA.

D, and a bit B...

The oil pressure would be lower, although how low depends on what your definition of 'Low' is - by the time it is hot enough to have a pressure issue, the engine would have already melted, in which case answer D becomes the overriding factor....

As usual, it depends..
 

Yes, true.

Lets consider.
A straight weight oil will tend to show a lower oil pressure, simply because of the oil composition.

Whereas, a multi-grade (Phillips 20W50 or 25W60 will show a HIGHER oil pressure, due to viscosity.

Yes, true and also, better cooling with the multi-weight, at higher ambient temperatures.

Surprise...surprise.

For an exam, the "Thought Police" will want B - don't let reality get in the way!

There is a point on which I differ. The fundamental is that the viscosity of oil changes (drops) with increasing temperature. The RATE of change is what differs between a multigrade and a straight oil. The multigrade will drop less viscosity than a straight oil as it gets hotter, but it will still drop.

The change of viscosity with temperature is measured by the Viscosity Index (VI), which is also a measure of an oil's multigradedness. The higher the VI, the more multigraded the oil. An SAE 40 (monograde) has a low VI while a SAE 20W50 has a high VI. The following chart shows the principle, where sample A is a monograde and sample B is a multigrade. As always with viscosity, note that the Y-axis is a logarithmic scale.
http://www.geocities.com/profemery/image004.gif

Oil pressure and viscosity are related in the sense that reduced viscosity means reduced oil pressure. In aircraft engines this relationship is complicated by various oil systems such as the pressure relief valve and viscosity valve or ventatherm. A multigrade oil (or a heavyweight monograde oil) might show a higher oil pressure during normal operation in some engines, but when the temperature rises above the ability of the system to adjust, then the oil pressure will drop. Answer B is correct.