Is it ok to change and flush the coolant while it is all warm. I've been on the highway for a little while so everythings probably warm and the pressure's probably up. Should I let everything cool down first? the car is an 83 300sd does anyone know the capacity? alos am i supposed to do 50 50 coolant water ands what shold i pour in first? thanks

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W126 1983 300SD 286,000 miles and ticking
Baby blue exterior Grey MB tex
Recent work:
Replaced air cleaner mounting brackets and heat shields
Replaced alternator, fan and power steering belts
Replaced positive battery terminal
Replaced negative battery terminal and cord
New Duralast Battery

My car needs work.