I ALWAYS change BOTH filters at the same time (10K miles). They some as a set when I get them and the secondary is the easy one to get to (on the engine) and it is a smaller micron filter so the primary on the fraim rail catches all but the small stuff. I would try the filters first as they are alittle easier to do, and always good PM. Fuel system repairs on the 6.0 is extremely expensive, so I am more than happy to replace $45 in filters.......