I switched over my new to me 1993 940 from dyno to 5-40 EU blend Mobil-1
last summer and have only put about 2000 miles. The oil was black and
I just changed it after 10 months.
I suppose the full synthetic is doing it's trick and cleaning out the engine.

According to Mobil-1 You get 10,000 miles or 1 year for the full synthetic.

This is a ball park number inflenced by the Mobil-1 sales department, so
if your oil looks OK maybe you can slide.

If you want to get technical, send your oil out for analysis and let them tell you.
My pal does that with his Volvo Penta deisel in his boat.

Another item in low mileage cars is the T-belt that will age over the years,
so worry about that more than the oil with an interference engine.

Cheers, Bill