More on Oil

So today, we have something like 6 degrees Celsius outside. I've just completed about 3 miles around town and I have measured the oil temperature using a meat thermometer (not the one that was stuck in the Christmas turkey last week), and we have 73 degrees Celsius.

Please don't stick it in the Turkey

If you use one of these though, you will need to stop the engine first. The RF interference driven by the HT cables is phenomenal and stops the thermometer working at all. I guess the thermocouple wires pick up the RF.

Now, looking at the oil data sheets, we can see that the kinematic viscosities are similar at 100 degrees Celsius. Plotted they look like this:

This gives us a clue, but we can't get too excited because there are only two data points per grade. The temperature/viscosity relationship is not linear. Look at this, plotted on a logarithmic scale:

This gives us a better picture. Notice that the viscosity of the SAE 50 is about 20 cst at 100 degrees C, which agrees with the data in  the first chart, but that at 75 degrees C the viscosity is about 40 cst. That should give us about a bit more oil pressure for the same temperature than the 20/50W we are using now.