Good question. I am not sure why but usually the outer joints go bad in my experience far more often than the inner joints.
That does not mean it is not an inner joint though. Just not nearly as high a probability in my opinion. Yet still in theory it should probably not be selective. First check on all four joints is the boots though for any signs of oil leakage.
The nice thing about adding a little oil and a new clamp is that it is cheap to do. I would use about 3-4 ounces of a fairly heavy weight oil for the test on each side. Most cv joints are lubricated with grease. Mercedes uses oil and I suspect that is why they last so long. . Grease sometimes changes in comparison as it gets old.