I'm sure there are a number of possible causes for your exhaust issue. I'll only mention the cause of the exhaust problem I had. The shop put too much oil in during an oil change (I think they put in 4.0 quarts when they were only supposed to put in 3.7). When the oil got hot during my next long road trip, it expanded enough to overflow a little out of the filler port and on to the very hot manifold. It was also winter so the cabin heat was on in the car and drawing burnt oil smells into the interior from the engine compartment for the whole trip. A quick check of your dipstick will reveal if that's your problem or not. I could also see oil residue around the filler cap.