Well,
All liquids become more thin (more viscus) the more heat you add to it. And since you are using a multiweight oil, one with polymers, its ability to change viscosity will also change with the amount of heat added.
When it is cold, the oil is thick and follows the 10-weight principle which means that it's thickness is able to change rapidly, and when it is up to operating temperature, it becomes much more thin and follows the 40-weight principle which means that it's thickness is not able to change rapidly (more constant, stable viscosity).
I am sure you already know this, but the point is that as oil is heated up it will become very thin. This however, is not a problem at all! The oil is designed to work and still lubricate your engine at this lower viscosity (thinner).
So having thin oil is not a bad thing, but what you want to be sure about is how sticky the oil is when its thin, or what is know as its film strength.
Next time you get your engine good and warm, pull out the dipstick and touch the oil at the end of the dipstick between your free hand's thumb and index fingers. Then let go of the dipstick, and touch your fingers together over and over. Pull your fingers apart really really slow and notice how far you can pull your fingers apart without the oil separating. This represents the oil's "film strength".
Good, Fresh oil will allow you to separate your fingers by almost a 1/4" before the oil separates. This is a very high film strength. But when oil gets worn out, its film strength decreases alot.
Do you follow me? Its kind of hard to explain I guess..
But this is how I check the life of my current oil in my engine. Most people go by the color, but even oil that "appears" clean may have already lost its film strength. And this film strength, or stickiness, is VERY IMPORTANT for the oil to stick to and cling to moving parts. If the film strength becomes too weak, then the oil may be thrown naturally from moving parts just from the centrifugal forces that are present on any rotating object.
You can try to use a single grade "straight-weight" oil, such as a 40W oil, which would resist its change of viscosity, but once it gets hot, it will be just as thin.
Regards,
BigMike
