I ran a wire from the Rally Design oil temp sensor I fitted in the turbo oil feed hole to the water temp sensor plug tonight. Went out for a drive. No temp display bars showing.
Kept driving. After 10 minutes the first bar lit up. Bingo!
After another 10 minutes the second bar lit up. Wahey!
Didn't get any hotter than that.
This suggests that the oil temp sensor has about the same resistance-temperature characteristics as the water temp sensor.