paint coating wax, ceramic coating?
 

what do you use products, for the paint and have you notice any mpg savings?

Seemingly, it doesn't matter what you put on your car. The airflow over the surfaces doesn't change just because it is polished.

Heck, a rough surface from dirt might end up better depending on where the deposits form.

But neither way will matter much compared to which way the wind is blowing, or other minor differences between drives.

Waxing your car makes a difference, but it is so small that you would never notice it.

To put it in perspective, an F1 car gets waxed before a race and even the panel gaps are taped to reduce drag. This yields a 0.05 seconds saving per lap. One lap is about 3 miles long, but races are typically around 65 laps so the advantage is about 3.25 seconds. Every little bit makes a difference.

Taping the gaps make a difference but the waxing is just for show. After the first lap all bar the car in first place are covered in muck and bits of rubber.

Sometimes during a tire change they wipe the front and rear wing clean of rubber debris, but they usually only have 2 - 2.5 seconds to do so.

Aircraft waxing - for example, the WWII P-51 fighter - was performed by the ground crew. Other searches considered, this may have added a very small speed increase.

The aircraft did not have the same exposure to shredded rubber, airborne dust, oil et al ad nauseum.

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