OP, I think you may have some misconceptions. Now, I've had plenty of "discussions" with Stevie about Amsoil, and I will always like PUP. But I think you need to realize that on a short OCI (yes, 7k is very short for a syn) all of the oils will look more similar than dissimilar. Amsoil SS is marketed as an extended-drain oil, which means it should maintain viscosity and TBN longer than your other oils. I have a feeling if you were stretching these to 15k or beyond, you would start to see much bigger differences.
IMHO, if you're staying 7-10k, use the Motorcraft blend and save some money. If you're in the 10-15k range, PUP is plenty. Longer than 15k, I would definitely give the Amsoil a hard look. Also, to make sure you're getting an accurate sample, make sure you've driven the car for at least 30 minutes at a good speed/load, then go home, wait about 10-15 minutes and drain it. This will help boil off any fuel.