Yes, it is. If you haven't been changing the oil filter, this is about the mileage that your oil will begin it's "sludge" building phase. I used to use one of the early wide range viscosity synthetics (Quaker State back in the '80s). At 10K miles, all looked well. By the time 15K miles came around, there was a thick layer of sludge stuck to the underside of the valve cover. You can imagine what the rest of the engine looked like. I had to do an engine flush to get that crud out.
Synthetics may be great for a long time, but oil filters aren't.