About 2 years ago, I installed the oil air cleaner and Aero exhaust to my 2001 Maraquis, C-12. The mileage improved about 1 MPG and I think the performance improved also. Coaches that use to pull away on hills, do not leave me behind anymore. The noise is not noticable inside the coach until you drive beside a concrete wall or use the Jake brake, I would recommend both additions.
Larry