DexronIII w/Mercon will work fine, that's what I have in both the 560 and the 4.5, or you could even go with a synthetic ATF, such as Mobile One ATF. No additives are necessary. Any brand name, and some "not-so" brand name are fine. Just recently there was a post about Wal-Mart brand ATF - actually a name brand in Wally World packaging.
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72
'02 CLK320 Cabriolet - wifey's mid-life crisis
2012 VW Jetta Sportwagon TDI...at least its a diesel
Non illegitemae carborundum.