Our engines, gas or diesel both have the same major exhaust product, water.
Leakage past the rings or other engine part will come in contact with the engine
oil. We have all seen the light colored oil foam on an engine that runs on
mostly short trips. Its mostly water. If engines do not get up to temp the water
stays with the oil or reacts with other elements to form worse things like acids
or solid sludge.

To get rid of the water oil must get hot enough. The only way to do this is for
the oil to get over the boiling point of water.... 212 F.    Oil will not
degrade signifacantly at this temp. But this does set a narrow temp range. Sure
oil is thicker at lower temps but if you dont flash off the water it just a
chemical reactor making stuff that will kill the engine fast. Most will say that
maybe 230F is a good upper limit. much higher and oil will START to degrade but
many stock engines run at 250 F and up and don't have problems. Oil coolers that
run the temps down too far are too big.

Many water cooled engines coolant thermostats don't even open at below 180 F.
Operating temps are often just below waters boiling point. If you try to cool
the engine much below this the thermal expansion figured into the
piston/cylinder gap will not make the best seal between the piston and cylinder
and less power will be produced.

So for long engine life 212 F to 230 F

Pawling NY

Cars I have: 93 Eurovan MV
                  75 Westy
                  73 Super

Cars I had: 89 GTi 16v
                 87 Syncro
                 83 Rabbit GTi
                 82 Vanagon Diesel (w 83 GTi engine)