If it is in limp mode the Check Engine light should be on and it would be necessary to retrieve codes stored in the computer of your car that would be the reason why the computer has decided to put the transmission in limp home mode.
I have this same exact problem. Except I haven't put any fluid into it and cannot check the codes myself. I bought a 2002 s430 a couple months ago off someone on craigslist and the car seemed amazing when I got it, then a week later the check engine light came on and it drives just.. kinda... like its distraught. it is most evident when its driven on the freeway, which I pretty much will not do knowing It cannot do that. I want to know how much this is going to cost. Did you ever get this problem fixed?
Yes I got mine fixed... the coil was the problem mine was having. I bought new coils for $1700 each and got a mbstar to reprogram the car and everything went fine. Just that the car became as fast as a jet then my kid bro crashed it. I will advice that you check the plugs and coils in yours. It's a V8 engine you have and that should be cheaper to fix compared with the V12 I used to have. Now I have the 2007 S350 and it drives so fine.