This discussion of oil and engine temperature relates to an idea I am
currently trying. Ideally as David Kao says, you don't want the oil to
be too low in oil temperature nor too high. 180F would be
about right. What I have done is install a second cooler sitting above
the tranny with a fan swiped from a Renault 5 and a motor from Subaru
(the only combination that would fit under there). Sitting in line
with the exit hose to the second cooler is an oil temp switch which
automatically turns the fan on at 180F. Ideally this cools the oil
down to  a manageable temperature above 180 but below 250 which is
dangerous for extended periods. Temperatures above 180 are necessary to
burn off any water that can ruin the oil.

Since I cannot hear the fan with the engine running  and do not have
an oil temp gauge, I have hooked upa small light to the dash which
comes on when the fan is operating. The fan operates mostly on long
hauls and up big hills.

The next step would be to install a thermostat which under  startup
and cold conditions would only allow the oil to flow to the second cooler wqhen
it reaches a certain temp. This system in my view would approximate
the kind of cooling you would get with a water-cooled vehicle.

There is one problem though, The volume of the xtra hoses and cooler
necessiates the addition of extra oil (I'm assuming- any ideas?)
rendering the dipstick useless (so to speak). At rest the extra oil
flows  into the pan and makes oil level determination difficult. Is the
extra oil really necessary??

DC

BTW the parts for the extra cooler including thermo and switch can be
had from Bergmann in Escondido.