All I can do is tell you my experience: My 300CE had Amsoil 10w30 in it from the previous owner, which is too thin for summer temperatures according to the owners manual. Consequently, I wanted to get the approved 40 weight in there, but couldn't find anything in synthetic in a 40 weight. So I went with Chevron Delo 400 15w-40, figuring it would do for the time-being until I could get my hands on Amsoil 10w-40. After the oil change I was surprised to discover that my valve-train noise was reduced significantly! What does this mean?? Well, I'm not sure. I have nothing against synthetics, but now I am pretty sceptical whether I should put synthetic back in this PARTICULAR car knowing what it sounded like with Amsoil in there - an arguably very high quality synthetic. Delo 400 is also a great oil, so I think I might stick with it FOR THIS CAR and forget about synthetics. Maybe you should do the same with your car.
'84 300D, 173k
'90 300CE, 163k