p&a-
As AOTW noted, the oil temp sensor is only detecting the local temperature at the sensor tip. I don't know what particular type of sensor was used on the examples you describe, but one common type of temp sensor is the thermocouple. The thermocouple uses two different types of metal conductors that produce a temperature dependendent voltage at their junction point. The junction point is usually at the sensor's tip. An oil temp sensor is usually mounted such that the tip protrudes some distance beyond the housing/manifold wall and into the oil flow, so that the bulk oil temperature is being measured rather than the housing/manifold wall temperature.
While your question about MGB oil temp readings is valid, I would suggest that we take a step back and look at the bigger issue of why MGB oil temps are important. All MGBs are essentially oil cooled devices. The friction heat generated at the gears and bearings is removed almost entirely by heat transfer to the recirculating lube oil flow. There is very little heat transfer directly from the bearings and gear meshes to the housing due to the limited thermal conduction paths. With regard to MGB temps, what is of primary concern are the operating temps of the gears and bearings themselves, and not so much the oil temps. But since there is presently no practical method to directly measure the operating temps of gears and bearings, we use the indirect method of measuring lube oil temps.
Hope that helps give you a better overall perspective of the issue.
riff_raff