I turned an eight bay selfserve in to a three bay Auto Repair Shop and a office/waiting room. The selfserve customers became instant customers with little to no advertising. I have been in business for over six years and things are good, the car wash revenue went down alittle but did not get cut in half.
If you are in a cold weather climate save your waste oil, buy a waste oil heater or boiler. I no longer use natural gas to heat the wash or the shop. I have considered heating the water with waste oil as well.
I would offer more then just oil changes.
A/c recharge
Transmission flush
Coolant flush
Batteries
Maybe even light mechanical work
Brakes, belts, ect.
Don't sell your self short.