I was thinking of replacing the thermostat but I thought of two advantages
with a cooler thermostat:

1. The heads would be at a lower temperature.
2. The thermostat opens up and lets 'heat' (albeit lukewarm) to the
passengers straightaway.

Does this make sense?
Are there any disadvantages?

Am I right in thinking that in a normal journey, the thermostat has just
one job which it only does once: waits until the engine is warm enough,
then opens to let hot water into the radiator and the two internal heat
exchangers.

Is there a disadvantage for the engine to warm up more slowly? My needle
gets reasonably quickly to the end of the minimum region and stays there
(25%) for over half an hour. After an hour or so it does climb up to the
middle, but often drops down again on downhill stretches. Surely by this
time the thermostat has already finished its job?

PS: I checked for air in the radiator by tilting the front up and opening
the radiator bleed screw and getting water. Did I need to open anything
else for this test?

Lionel Tun
84 GL, 183k miles
lionel@ark.prestel.co.uk