Oil tank breather

That's interesting about the diamond honing. That could certainly explain why my new rings never wore-in and may partly explain my high oil temperatures. However, that doesn't explain the continuing high temperatures in my new engine where there is no oil consumption.

I am assuming that it is either oil flow, oil cooler, coolant flow or coolant radiator. I've elminated the latter and the coolant temperature and CHT look OK. I'll be checking the oil cooler next.

I would take a good look at the airflow for the cooler. Creating enough suction at the back air exit of the cowl is the key. Also do the internal oil hose diameter and fittings meet the guidelines of the Rotax installation manual?

Nothing else has changed over the life of the airframe. In the UK you cannot make changes to your aircraft, it is as per the original approved design. When I bought the aircraft about 10 years ago, the oil temperature was lower, so this is a relatively recent development.

I can only imagine that it is something like a hose that is collapsing when it gets hot or a bocked tube in the oil cooler.

What is the average oil temperature in climb and in cruise?

On a warm day

Ground level ambient temp = 27 degC

Cruise alt = 2000 ft

Temp at alt = 23 degC

RPM = 4900

ASI = 100 mph

Oil temp = 120 degC

I don't tend to let it get any hotter than that. I can only climb to about 2000 ft before I need to level out and cool the engine before climbing higher.

There doesn't appear to be a linear relationship between the ambient temp and the oil temp. Compare the above with the following.

Ground level ambient temp = 16 degC

Cruise alt = 2000 ft

Temp at alt = 12 degC

RPM = 4800

ASI = 100 mph

Oil temp = 95 degC

So a 11 degC reduction in ambient results in a 25 degC oil temp reduction. That really doesn't sound right if the cooling was just down to the oil cooler. It suggests that some other mechanism is at work.