According to what's been posted here by pros and at one other MB car site, the valve guides were updated in 4/89.
If your spark plugs are dry and you're not leaking noticable amounts of oil, I wouldn't be alarmed about 1 qt. every 1,500 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.