I have a 2016 F56 Cooper S with 27k miles, and just noticed a "trail" down the windshield from the rearview mirror base. It appears to be the rain sensor gel mentioned above. I have the CPO warranty until May 2021, so will watch this to see if it affects the rain sensor. I will update here after I seek repair by the dealer.

 

Well that is interesting...lol. Before reading this thread if someone had told me they needed to change the review mirror gel pack I would have to told to not forget the muffler bearings and blinker fluid.

 

Last year dealer replaced the rear view mirror after 2 unsuccessful "rain gel" repair attempts