I hate these oil posts, but will respond anyway.
You have a '91 model W126 car. Same as me, just a bigger engine.
I've used conventional oil for many years and have taken more than one car past 300K with it. I used synthetic oil once(don't ask me why) and all it did was create leaks. It's known for doing this; however, in it's defense, it may be less likely to do so in the cars it was intended for - newer models.
Use a Mann, Mahle-Knecht; Hengst oil filter and the prescribed amt. of oil(conventional in my opinion). Do as Larry Bible suggests, changing it hot and often. The oil in my MB gets drained every 1500-2000 miles.
1991 300-SEL - Model 126
M103 - SOHC
James Madison - Oppressors can tyrannize only when they achieve a standing army, an enslaved press, and a disarmed populace.
Liberty Firearms Training - After years of trying to regulate California criminals with a never ending stream of gun control laws they have still not given up with the belief that if they make something a crime that criminals will miraculously start obeying the law.