If you ask a racing engineer where you should measure oil temp they'll most likely tell you to measure temp and pressure of the oil that is being sent to the bearings. But that is hard to do unless you have a dry sump setup.

Oil temp in the sump isn't a bad way to go for a wet sump engine. On the dyno we either stick the oil temp probe into the gallery or we stick in the remote filter.

On a street car I don't think it matters where you measure the oil temp. Just find a place that works and then keep it there. If you measure at a remote filter or the back of the block you might be a few degrees lower than the temp in the pan but I wouldn't worry about it.

What surprised me when I hooked up an oil temp gauge was how long it took for the oil to get up to temp. A street car doesn't get the oil hot until you are on the freeway for 10 minutes or so.