Can I use regular gas once in a while
As long as your car does not ping it won't hurt anything. You will probably just lose a little power.
Not really, 91 or higher should be used, its a higher performance car...
If your wanting to save on gas bill, in the long run the best thing to do is buy a cheap beater car or truck to drive when you want good gas mileage or its raining or your going to work ect. Keep your sports car nice.
If you want to damage your engine, sure.
You won't damage the engine. You can use 87 octane as F_O_R suggested. You will just get lower MPG and the knock sensor will take care of the rest. 91 is recommended. So you can put in 93 one week, refill with 89 the next, and back to 93. So essentially you are mixing. Is 91 or 92 octane sold by you? Calculate the cost if your concerned about saving a few dollars.
Looking for a Used 5 Series in your area?
CarGurus has 516 nationwide 5 Series listings starting at $3,800.
Search BMW 5 Series Questions
BMW 5 Series Experts
Related Models For Sale
Used Cars For Sale