A month or so ago, I replaced an inexpensive muffler (dual exhaust) that was in pretty bad shape with a flowmaster 40 series (dual exhaust) on my 2000 Lariat, 5.4L truck. Before replacing the muffler I was using quite a bit of oil. Since changing the muffler, I have had very little oil usage. It is strictly from the back pressure now being applied, by a good quality muffler that has made it stop?