Anyone figure out the difference in these yet. I forgot to look at these at the dealer the other day. None of the other parts stores have the 500S. My friend ordered the Ford Racing filter and it is a version of the 820S. I assume the difference is like the old FL-1A and FL-400, the 400 was a shorter version of the 1A. If the 500 is a shorter filter then it would be better using the 820S but any other reason I will look to replace with the 500.