If it was my car I'd prefer the Schaeffer over the Castrol. The Shaeffer 9000 is a true PAO synthetic. There should certainly be no worries or concerns over switching. If it's easily available I say lucky you, lucky car, go for it!
Some might accuse me of having a bias in favor of Schaeffer- OK, they might be partly right! If so, there's good reason for it. To see why, search the UOA section using my name of member # to see results from at several runs with Schaeffer 7000 syn blend 10w30 SL.
Schaeffers' 9000 is not a true PAO. It is a group III/group IV blend. I am not slammering this product. It is an excellent oil. It is one of the best values in oil today.