I accidentally added oil to my coolant reservoir. I have a BMW 325i. The oil light came on and I figured my oil may be low. When the oil was added the oil light warning went off. However, the service engine light came on. It was at this time, I checked the manual and discovered that although the cap where I added the oil stated use ATF oil only, per the manual this is the cap for coolant. Please tell me just how much trouble I'm in and what the cost will be to fix it? Thank you!
The very first thing that you should check is whether you actually made a mistake and, if so, which one.
A cap that says "ATF oil only" is for automatic transmission fluid. If you're adding engine oil to the transmission, that's going to make your transmission very unhappy. It is fixable: you'll need to drain the transmission fluid and replace it.
If you actually did add oil to your coolant reservoir, there was a similar situation discussed here:
Essentially, the coolant will need to be flushed and replaced. This isn't going to kill the car but it's best to keep driving to an absolute minimum until you've flushed the system.