nothing worse than mixing medals!
anyway if you don't want to mess with the plug i would just find a hill point the nose downward do what your going to do.
point it up hill check your owners manual on capacities and use the MB coolant not going over the % of mix required. my guess is a bit to much is better than not enough.
make sure you burp the system with the heater on & are getting heat.
i have bronze pool pumps out there and after 2 years of salt air and pool water leaking past the seals they never come apart even with the bolts removed!
the case, minus a few cans!