Experienced something cool today: there were Starlings flying about finding breakfast, and I decided to try some BIF. With the 100-400 II, camera in all-points AI-Servo, I was able to get the camera to lock on and track them, and actually get in-focused shots. Never had this kind of prowess with the 5D4, it always seemed just a small step behind, so I really haven't been one to even attempt BIF. Frankly, I'm blown away.
The oil spatter issue, I've read about it as well; since I'm a CPS member and a clean is a quick turnaround due to Canon being relatively close, I am not sweating it. Yes, it sucks, especially for the money spent, and it was actually part of what initially kept me from wanting to keep this camera. But, if it isn't affecting my photos (I rarely shoot smaller than f/8) I can live with it. In my book, the benefits outweigh this inconvenience.