All I can add to this depbate is that if your t-stat won't close completely you will run too cool. I just swapped mine out today. Before the swap, with half the radiator blocked with cardboard (a temporary measure to try to raise the temp) it never got above 70*C (160*F) in city traffic, and spent a lot of the time down closer to 50*C (120*F) on city streets.

After putting in the new one it runs right about 85*C - 95*C (185*F - 205*F) on city streets. Haven't had it out on the highway yet. I was getting about 24 mpg around town before. I expect that will go up 1-2 mpg now that it is operating at the more efficient proper temperature.

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1984 300 Coupe TurboDiesel
Silver blue paint over navy blue interior
2nd owner & 2nd engine in an otherwise
99% original unmolested car
~210k miles on the clock

1986 Ford F250 4x4 Supercab
Charcoal & blue two tone paint over burgundy interior
Banks turbo, DRW, ZF-5 & SMF conversion
152k on the clock - actual mileage unknown