While not exactly common knowledge, this "secret" is hardly unheard of in the SAAB 9-5 community, but I figure it doesn't hurt to give more coverage to the "enhanced" alternative to the stock oil filter.
The oil filter for a 2003 Chrysler PT Cruiser Turbo fits on the SAAB 9-5 and the filter extends about two inches longer than a stock 9-5 filter. That is two extra inches of filter material to help guard against oil sludge.
Here is proof that the filter fits, taken right after installation. Ignore the oil drip, that's just from the oil I primed the filter with before installing.
The filter is a Motorcraft FL-400S or Fram 3600... or you can just look up the oil filter part number for a 2003 PT Cruiser Turbo. The precise year of PT Cruiser isn't critical, but it must be a turbo model to get the extended filter. I suggest the 2003MY because I know looking up that year will get you the correct part number... because that is exactly what I did.