As you have seen in the comments there is a discussion about
excess oil in heads down cylinders and fuel taps. The theory is that with wing
tanks (high wings) fuel continues to slowly flow through the carburetors due to
the gravity feed and that the oil is deposited in the cylinder head while the
fuel evaporate or disappears elsewhere – if there is no fuel tap prevent this.
As I’m normally not a friend of fuel taps (risk of forgetting
them, point of obstruction, leaks…) I had reduced them to a minimum.
I have reconsidered and have now installed a fuel tap on the firewall,
operated directly on the dashboard. I did this some time ago but as the weather
has been dreadful most of the time and particularly so when I had the time to
fly I have not yet been able to test this.
As soon as I can confirm that this is actually solving the
problem you will be the first to know!