At my last oil change, it took a fair while time for the oil cooler return line to heat up, long after the oil temperature gauge is reading up to tempurate in the tank. Exactly as you describe. The return was stone cold for ages and suddenly went "oh yeh, that's hot" hot in the fender and gradually worked back.
Personally, I really hate letting the motor idle that long. Go and drive it hard for half an hour or so and then check the return line.
It's my take that until all the oil in the system - including the stuff in the cooler and the return line - is up to temperature, you shouldn't check the oil level - because you'll get a false reading (too low).