A cold engine, idling during warmup, tends to condense fuel vapor on the cylinder walls, washing off the oil film. There are other reasons - low cold oil circulation for one. Water condensation caused by cold air drawn in by the PCV system condensing on cold parts is another.
Most automakers and car magazines recommend that you start the car, allow the engine controls or carburetor to fast idle the engine for 15 - 20 seconds, then drive away normally. The engine warms much faster as does the oil. Don't use full power until at least 10 miles have been covered.
Your Deville and my STS will "fast idle" at about 1200 rpm for about 20 seconds or so, then drop to 900 for a minute or so, then to the normal 650. (Recently, we've seen [in here] that some of these cars seem to idle at around 1,000 when it's really cold. This may be either a PCM control function or due to cold oil drag. Anyone's guess.)
For a demonstration of cold oil, say 5W-30, pour a pint into a Mason jar, cap it and stick it in the freezer. Let it chill to 15 degrees or so. Now shake the jar. Gooey, isn't it ? You want that oil to warm as fast as possible, and light load driving does just that - safely. Just because the heater blows warm air does NOT mean the engine is properly "warmed up".