Here is how the viscosity rating is explained. The following is from the link that I have provided at the end of the quote. Also the operating temp of the engine is measured by water temperature and not oil temperature. Oil temps are almost always higher than the water(cooling) temp as oil is only cooled from the heat exchange effect created by the water cooled system. That is unless there is an oil cooler installed.

Quote " Oil weight, or viscosity, refers to how thick or thin the oil is. The temperature requirements set for oil by the Society of Automotive Engineers (SAE) is 0 degrees F (low) and 210 degrees F (high).

Oils meeting the SAE's low temperature requirements have a "W" after the viscosity rating (example: 10W), and oils that meet the high ratings have no letter (example SAE 30). An oil is rated for viscosity by heating it to a specified temperature, and then allowing it to flow out of a specifically sized hole. Its viscosity rating is determined by the length of time it takes to flow out of the hole. If it flows quickly, it gets a low rating. If it flows slowly, it gets a high rating.

Your engine needs oil that is thin enough for cold starts, and thick enough when the engine is hot. Since oil gets thinner when heated, and thicker when cooled, most of us use what are called multi-grade, or multi-viscosity oils. These oils meet SAE specifications for the low temperature requirements of a light oil and the high temperature requirements of a heavy oil. This is the best of both worlds and is accomplished by using special additives to increase viscosity with increase temperature. " Unquote

http://www.autoeducation.com/autoshop101/oil-change.htm

I hope this helps clear up some of the mystery that seems to surround what the viscosity numbers mean and when they are achieved as engine oil temps rise. 10w40 oil would have a rating of 10 at zero degrees and 40 at 210 degrees! The viscosity of the multi grade oil rises with the temperature of the oil and increases the oils wear protection.