Dino oils tend to shear a bit at first, and then as time goes on, they start to thicken back up b/c the lighter ends of the oil, basically, start to 'boil off'.
You SHOULD be changing dino oils before they start to thicken up - which is why most UOA's you see, the oil is slightly thinner than the specified grade.
Long drain synthetic oils are a bit different - they are built to be stout enough that they can 'take' some of this thickening, and still protect well. So you will see long drains of syn oil (10k+ miles) that are at the upper end of their visc. grade, or into the next one, and the oil is still good. That is why you want to stay in the specified grade for extended drains - you will most likely be going up one oil grade anyway.