I can't speak for the 240 oil temp gauge but do recall that P1800 cars with working oil temp gauges did not show anything till the motor was running for a while. I installed an oil temp gauge on an air cooled VW bus which had a scale from 140-300 degrees. The gauge did not start moving until I had a good ten minutes of highway driving under the wheels. I don't think you are seeing actual oil temp on your gauge after only a few minutes of running--more likely heat transfer from the block as others have suggested. -- Dave

With the guage sensor in the spot you mentioned, you are probably seeing the temperature of the engine block at that location, as conducted through the fittings. Since the oil is not actually flowing in good volumes past the sensor, the mass of metal there will tend to dampen out temperature fluctuations in the oil.

Engine coolant (block, head) temperature is rather stable, once warmed up, but creeping along in warm weather traffic, or idling, will bring the oil temperature up, as there is little airflow over the sump/pan to cool it. However the radiator fan will keep the engine temp pretty steady, and that's what I expect you'll see. That's the theory - let us know what actually happens.
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Bob (son's 81-244GL B21F-M46, dtr's 94-940 B230FD, my 83-244DL B23F-M46, 89-745(LT1 V8), 98-S90, 77MGB and four old motorcycles)