That doesn't surprise me one bit. I use Amsoil in my car and like their products a lot. Last time I had my car at the dealer for xx mile service I had them change the oil since they were going to charge me for it whether they did it or not. I left the Amsoil and a filter in the trunk with instructions to use it instead of the manufacturers stuff.
When I picked up the car I was told that they used the Amzoil but the service manager said they could not use a nonmanufacturer brand filter. He hinted that it was a warranty issue and they didn't want to have a problem down the road. What a crock of shift. They used the nonmanufacturer's oil for Christ sake.
There are a lot of synthetics out there and all of them are better than the best dinosaur oil. Just one man's humble opinion.