The simpler [and substantially cheaper] way to mount an oil temperature guage is to buy a fish tank temeperature guage with wire and sender bulb from Maplins and zip tie the sender bulb to the outside of the oil tank, and lead the wire through the car, mounting the guage itself on the dashboard wherever you want it. The cost is less than £10, it is easy to do and you do not have to drill holes.
The downside is that you only get an approximation of the oil temperature, and if it goes over 60degrees the guage cannot cope, but then it is getting quite hot, and when drivng the temperature remained around 45 = 55 degrees.
It certainly gave me enough information to know what temeprature my oil was.
