First of all I do think an oil temp. gauge is a cool instrument. Second I totally agree that the drain plug location seems wacko. (You can get that from Volvo if you want)

But let's examine the point of an oil temp gauge - I really don't think the point on a NA car is too see how hot your oil gets - no doubt EXCELLENT information but if your oil is TOO hot, what is your H2O temp. gauge saying? It's too late. Good for comparison - and a good reason in itself to get the gauge. However, I have a different take (and a point) I think the best use of an oil gauge is to tell you when your oil is warm enough to drive like a maniac (if so inclined) - it's a better gauge than the H2O temp guage for this because the coolant could be warm but the bottom end oil has not fully warmed....and with bottom end warming comes less condensation, better engine and oil life.

My point: you want to measure where your oil will be the coolest/cooler. A remote oil filter is a good place. The back of the head is OK. The bottom of the pan is OK if done safely.

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http://www.fidalgo.net/~brook4/oilslubesfilters.html