For the brake fluid my 2018 E450 says to use DOT 4 LV brake fluid. The LV means low viscosity. Both caps will use the same fluid.
As for the cap coming off, check around the opening of the good one that the cap stays on. There should be 3 little tabs around the circumference of the opening. Maybe those are missing from the bad opening because someone tried to pry the cap off instead of twisting it?
2018 E450 Forester 2291s