So I fitted a thermocouple sender temperature indicator, (i.e. pretty accurate), with the thermocouple about 1 inch below the oil level in the oil tank, which, after a number of journeys told me:-   
  
 Constant 65 m.p.h.    95°C   
 Constant 70 m.p.h.  100°C   
 Good belt 100 m.p.h. *  120°C   
 Around town, air temperature 20°C    95°C   
* on private roads, of course   
  
I asked the oil companies what happened to multigrades at 120°C +, (given that the oil returned to the tank is a mixture of cooler oil from the sump and hotter oil from the head) and also what temperature oil should be run at. I got no clear reply. However, this produced the information that the oil temperature in a car sump is, desirably, around 85°C. So, this made me decide to put in an oil cooler, and an oil thermostat, with the following results:-   
  
 Constant 65 m.p.h.    80°C   
 Constant 70 m.p.h.    85°C   
 Good belt 100 m.p.h. *  100°C   
* on private roads again, naturally   
 
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