Peter's last post says alot.
I more or less have the same vehicle as you; mine is a 1991.
I was using a qt. every 1000 and a new head gasket and valve stem seals cured that. I'll assume the people who did your valve job did indeed change guides, seals, OEM head gasket and also replaced the infamous U-Gasket up front. If all of that was done, you have a leak.
The 103 motors are "leakers".
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.