I had my 13 impreza do this after first oil change. They put some additive in the oil and at that time I had the mesh grille installed. Car hasn't been the same since.
Loss of MPG (2-4 at least avg MPG) and when I first had it happen it was like it was running on bad gas. Put in higher octane fuel and it went away for 100 miles or so and came back. It happens at lower speeds, when idling, and ehen the car is warming up for the first 30 minutes or so.
Took to a different dealer, the noticed the oil thing and it started only doing it when the ac was on or warming up on a hot day but never nearly as bad.
Tried to show the dealer, but they can't replicate it because I have to get my car serviced on Saturday early in the morning.
Next oil change is today, we will see what happens, but glad I found the thread and to know I'm not the only one.
Anyone else have additives put in their oil?