I experimented with this and my scangauge the other day.
I had assumed idling at elevated RPM, due to no load, would be much less expensive than driving at those RPM on the interstate.
NOPE.
Floored, hitting the 4000 RPM governor, my Duratec-equipped Sable was burning 11 gallons per hour.
At 3000 RPM it was burning 2.5 GPH.
I can cruise at 55 MPH on level ground all day for 2.0 GPH.
On the up side, my coolant temp appeared to go up about 3 degrees F per second, rather than 1 degree.
I did this as an experiment, and only for a few seconds, and in forty-degree F weather. Plus, due to my bankruptcy, I can turn this car in to the bank at no penalty to me if it throws a rod.
After 30 seconds, I would be willing to up the idle to maybe 2000 RPM to warm the car up until it was ready to break the ice on my car up. I do use 20-weight.
In the Cleveland area, some days the ice is thick enough that I risk breaking my scraper and windshield scraping it off.