I have Mobil-1 synthetic ATF in my 400E as well, no problem at all. I regularly check underneath for leak, there is no leak from the transmission.
I have been using Mobil-1 synthetic ATF in all my cars, it's also in the transfercase and P/S pump in my 4X4, I am very happy with it.
99 BMW 540i 6-speed 110K Km
03 SAAB 9-5 wagon 80K Km
92 400E (Sold) 245K km
Still missing the days with the Benz, it kept me busy.