what is correct atf fluid on 1990 Mercedes-Benz 420SEL

what is coreect atf fluid for 420 sel and where does it get filled once drained

The Correct Fluid for your German Made car Is Pentosin ATF 1. Made specifically for your car and lasted a long time between service intervals changes other fluids work but not as good. Electronic components such as Converter Clutches really respond to the Synthetic properties Of Pentosin ATF 1 No. A 0019892203. Info from My Mechanic In Germany. P.S. Can be purchased at your Local Parts house.
2 more answers
Dextron ATF works just fine and you fill it in the dipstick tube.
dextron or dex/merc3 ,$4 /quart as per my 20 year mechanic 1989-1990