Supposedly it does not make seals shrink. The first synthetic oils did that, because they screwed up. Now they formulate the oil to not cause shrinking or expanding. I don't think the seals will be affected themselves. But if there's already a crack, or a bad seal, then the gunk that is plugging it up now will be cleaned away exposing the leak. Cleaning is good, and long term is good for the engine, but the leak would need fixed, I just want to be prepared.

Mixing synthetic and regular oil would be pointless. I'm getting this oil for the extended drain, if I mix regular oil, I'm back to 3,000 mile oil changes.

Response from Amsoil tech-

"We certainly understand the initial reaction of those who are surprised at the consistency of synthetic lubricants. Outwardly they do seem “thin” in comparison. All petroleum oil has a certain look and pour rate that we are all familiar with. The molecular structure of synthetic oil is such that the pour characteristics are much superior and makes them appear thin, but actually the high temperature protective film strength is many times higher than petroleum. It does take some getting used to."

I'm not worried about the oil itself, I'm more concerned with the present condition of my engine.