Leaving a car to idle isn't a good way to warm it up to operating temps. Best way to warm up a car is drive it, keeping it off boost. And the temperature gauge, if you're talking about the standard ones in the dial pods is water temp. That will read warm pretty quick, your oil won't be. When I had a etb digital dash in my 106gti, it had a proper oil temperature gauge, and it was only just up to operating temperatures after about 10 miles of sitting at 60.