I have the Rotax Franz type of thermostat which mounts directly across the top of the oil cooler. More common in Europe than in the US. It's been on the 912ULS for about 300 hrs. On recent flights my oil temps have gone from their usual 90 C to 110 C and even hotter on a climb.

As nothing has been changed on the motor or cowling I can only presume the thermostat is no longer diverting hot oil to the cooler, or it delivers insufficient oil to the cooler. After flight, when I put my hand on the cooler it is barely warm.

A new Rotax Franz thermostat is nearly €400.

Just checking my thinking on this: Option 1 is to remove the cooler and thermostat, flush them out, re-install and see if the problem is solved.

Option 2 is to remove the thermostat altogether. It does not seem to make a huge difference to warm-up times.

I would really appreciate any thoughts or shared experience. Kerry

This link shows the type of thermostat: https://shop.franz-aircraft.de/accessories/5/