I recently had a similar issue with the M54 engine that was a bad CCV.

Not sure if this applies to the V8, but on the M54 while it is running if you try to pull off the oil fill cap and experience a vacuum that will usually indicate a failed CCV.

The vacuum isn't slight either, I had to really work to get the cap off.

Again, take this with a grain of salt though as I'm not sure it applies to the V8.

If it turns out to be the valve stem seals have you looked into what a rebuilt or low mileage motor would cost vs fixing the seals?