Like others have said, the profit margins would be very slim. Most people will just go to a quick lube because it usually only takes about 10min anyways.
As Scott said, some cars are a real PITA! For example, all Audi, VW, Mercedes, BMW and some Saturns, older GM trucks (4wd), Nissan PUs, Toyota PUs, Toyota cars, some older Caddilacs and thats only the ones that stick out.
Also think about how much stuff you would use to do the oil change. Say oil on average about 4 qts @ $2 per qt and filter @ $5, then you would need either degreaser or brake clean, and rags, so lets say about $15 per car is the cost. So is it really worth it?
Oh ya and you would need tools too.
Tools:
Wrenches (metric 13-22)
Ratchet, extensions and sockets (8-17)
Allen heads (not sure what size)
Oil wrenches
Thread cleaners
Re-threaders (if you strip a plug) and over sized plugs
Temp. plugs (just in case you really mess up)
New oil plugs
New gaskets
Filter sockets/cups (for canister filters and others that are only acessable with a filter cup) (adjustable would be good too)
Oil reset tool for BMWs (not cheap)
Books that give quantity, oil type, etc.
wrags
degreaser
brake clean
washer fluid (if you want to offer top offs)
powersteering fluid (if you want to offer top offs)
flashlight
funnel
Coolant (three types, green, dexcool and chrysler/ford coolant)
Tranny fluid (maybe, if you want to offer top offs)
Catch trays (at least two)
Gloves
Fender cover
Burn sleeve (you'll want that for toyotas that are hot)
Then you would need air filters, oil filters, oil, tranny fluid, gear lube, etc.
BTW I work at a quick lube.