When doing the shake test, let it sit for a while. If you have some murky, dark colored stuff in there now. It might be a good bit of goo. I tested a GL truck last fall by pulling a bunch out of the master, mixing in some BFS, shake and let sit. 30 minutes later and I could see no lines. The next morning though, we had 3 layers. BFS brake fluid, DOT 3 brake fluid and goo on the bottom.
The truck had DOT 3, we did a complete flush and got all the gunk out to make all 4 brakes work. I personally have been won over to the dark side and want BFS in every vehicle I own that doesn't have anti lock brakes. ABS doesn't get along with BFS in US made vehicles.