Re: 3.8 L knock
That engine is rated for 87 octane, so the dealer is literally full of crap. If he's been running 91 for a while, the engine will knock for a bit when it transitions back to 87 as the computer readjusts the ignition timing to compensate for the less resistant fuel. This happens anytime you switch from high to low octane and vice versa. It can take a while sometimes, but ideally the computer should be able to adjust.
If it keeps having problems, however, there is most likely a more serious problem with the engine itself. It may run on the 91 for a while, but its only a matter of time before the problem worsens.