I have done both, the interior and exterior coils on a six year old unit.
The interior uses a soap like a degreaser something like totally awesome, purple
Power, or simple green. It was for coil cleaning so it was overpriced. The exterior uses an acidic cleaner which foams and gets rid of what little oxidation there was. You hose it off to stop the foaming. Also a purpose specific cleaner.
Looking at the end state neither needed it.
Edit to add: it did not make one bit of energy savings, judging by my energy monitor which takes 10,000 samples a second.
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