It can be easier to make sure there is no junk in the engine bay, and that the foam is in properly and in good condition , as if the foam is falling out, its going to be leaking hot air from underneath too. I also know from experience that a bit of foam about the size of a decent mesh hole about 1cm across is still enough to unbalance the fan.
That hill is a steep bugger ...remember coming down it after i purchased by bus from Cirencester ,and how alarmed i was of the efficiency of the brakes . That was the first job i did on it when i got back home .
Buy cheap meat temp probe, dip in your oil and you will know if it is running hot on next test run. Sent from my SM-A405FN using Tapatalk
P⁰ You could get a Bluetooth one or extend the wire on a cheap remote probe meat thermometer. Just a bit of screened cable. All the way to the dash.
Is the thermostat opening the air flaps. Does it have a thermostat even A quick (I don't have an oil temp gauge) check of if a type 1 engine is too hot is remove the dip stick after a run. IF you drop it because its too hot ..then your air cooled engine is too hot.