You mentioned valve job. I can't help but wonder about the quality of the work, especially what was done to the cyl. head and if it was done correctly. Sounds like somethings getting past seals/guides. Which one - who knows?
Unless your oil change regimen is terrible or the car was run low on oil or too hot, too often, I wouldn't suspect rings.
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.