I just received a letter from MBUSA stating "Mercedes-Benz now recommends pure synthetic motor oils". And it also quotes "Based on vehicle performance testing, our engineers have concluded that for vehicles equipped with Flexible Service System (FSS), continued optimal engine performance at service intervals called for by FSS can only be maintained through the consistent, long-term use of pure synthetic motor oil."
"For that reason, pure synthetic oil is now the only engine lubricant recommended by Mercedes-Benz for 1998 and 1999 model year vehicles equipped with FSS. While conventional oils deliver reliable performance, pure synthetic motor oil is engineered to flow faster than conventional oil, thus lubricating your engine more efficiently. Switching to pure synthetic motor oil at your next oil change will allow your engine to perform at its peak."

I also received a Factory Approved Service Products Booklet October 2000.
The following are the MB Approved Synthetic Engine Oil:

Mobil 1 Tri-Synthetic 15W-50, 0W-40
Valvoline SynPower Full Synthetic Motor Oil 5W-40
Castrol Syntec 5W-50, 10W-40
Castrol TXT Softec Plus 5W-30
Shell Helix Ultra 5W-30, 5W-40

So this is it, I hope this will help to put an end to the debate on whether to use synthetic or not. However, the letter still maintains the position to continue to follow the FSS recommendation.

Car Nut
99 E430 Sport