Yah these cars are very sensitive and do require their own antifreeze , what i would do if i was you , would be taking it in for a coolant service by a BMW dealer , $200 bucks they will flush and clean all the contamination ( if there is any ) etc and let you know if there's anything wrong . Just my .2c
Coded, BMS S1 431whp/479wtq 93 Oct /Run Conditions 91*F
Options: Camera Pkg, Convinience Pkg, Driver asst, F&R Luxury Seating, Prem/Sound, Rear Entertainment, Sport Pkg, Massage rear seats, Ceramic Controls, Active Cruise, SmarthPhone Int., Night Vision, Head-UP Display, Sat, Leather Wrapped Dash&Doors.
(N54 E82 135i) JB4 ISO FBO, 434whp & 470wtq / Run Conditions 90.28 *F