It's funny how eventually this thread get revived as owners start to experience the same issue with their 550i. Adding to the list of what can go wrong in 2020 for my car, I just noticed the leaking return coolant lines at the T and noticed drippings on the alternator. This is one heck of an involved DIY, but I'll definitely tackle it myself in upcoming months before I go back to my normal commute. Likely end up changing a crap load of other parts - oil return lines, water pump, aux water pump - and whatever else I should while going through that massive disassembly in the engine bay.
