When i got a quote from bmw is was a small fortune, i would inspect it first if i was in your shoes.

Get it on a code reader if no joy take the sump off check the filter and pan for debris if things dont improve if its ran low on oil and overheated you could have damaged the clutches and they brake so you maybe able to spot this when you take the sump pan off.

Also another pain is not knowing the exact oil whats in it if its been out before its about 8-10 liters iirc (inc torque converter) so its alot to replace.