Why don’t I see any reference to the maintenance schedule listed by Saab in their owner’s manual. That is the first place to go for the answer to this question. Always remember it is the based on time or miles whichever comes first! Never do less.
If it were mine, I would take a look at the cost of having the dealer do it, since they are not doing it for free if they are charging you for oil, vs the cost of having someone else do it. Are you really saving anything? Remember you are not required to have it serviced by a dealer as long as you maintain proper records of the work.
It it were mine and considering the sludge problem that Saab has had, I would make sure it got twice a year or very 5,000 miles. I would also use a synthetic oil that meets all the specifications listed in the owner’s manual. If letting GM do it once a year is economical, then let them, but I would do an additional one.
That said, my car does not have a known sludge problem so I change mile oil at the manufacturer’s recommended 10,000 or once per year, using an oil meeting its specifications. I do it myself. I do believe that normally the manufacturer’s recommendations are fine. It is hard for many people to accept those long oil changes, but today’s engines and oils are far different than they were in the 60’s when I bought my first car. Today, even with longer oil changes, very few cars end up in the with engine damage due to oil related problems, if they have followed the manufacture’s recommendations or if they had a design weakness.