The brake booster should never have fluid in it unless the master cylinder is leaking. It's always possible for the fluid from the previous leaking master cylinder to still be leaking out, however since you notice the fluid level dropping in your reservoir, I'd suspect the new MC as your likely culprit.

Oil doesn't belong in the brake circuit ever. It can cause problems with rubber seals and can wreak havoc on ABS pumps.

The oil-contaminated MC should have never been installed on the car without a teardown, flush, and thorough cleaning.

Unmarked containers of fluids have no place in anyone's garage. Buy fresh brake fluid before doing the job and you have no chance of mixing fluids or contaminating the system.

__________________
Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]