well if that's all you can do...
BMW specifically says premium. I believe. I really don't know, but I am sure you will be fine, but the car may be rough idles and throwing some timing.
If you have to, I wouldn't get on it and drive like a grandma.
5 gallons of 87 would be approx. 90.7 octane. with your 8 gallons of 93.
Which I run 91 here in Cali all day long. without my tune.