I cleaned mine last year, and it was really dirty!!! It was used to heat the garage, and I took it outside and sprayed through the exterior with water, and not much came off. I do a good bit of welding/fabrication/painting in the garage, so it wasn't going to come off easy. I sprayed mine with Purple Power, let it set for about 5 minutes, and rinsed it liberally with warm water. I know the Purple Power can be corrosive to aluminum, but I did my best to get it rinsed clear. I'm not sure the efficiency improved, but it sure looked better. I also installed a filter frame when I put it all back together. No issues so far.
While I had it apart, I did flush the inside as well. I filled it with white vinegar and let it soak, then a good rinse. Much improved!