As to the valve guide portion of your question. You did not say which motor you have. I am assuming that it is a 2.6, if so:
The 2.6 is an M103 engine, little brother to the 3.0 engine in the 300E. I have seen a number of posts claiming valve SEAL problems on these engines. I have an '88 I bought used at 89,000 miles with very thorough records. In the records is a receipt for replacing the valve seals. If there was indeed very high oil usage before they were replaced, I would be amazed.
In fact the engine has done so well, I wonder if it was an independent shop scamming the unsuspecting owner. There were alot of suspicious, expensive repairs done by the same shop. There are many, good, honest shops out there, but there are some scam artists too.
Anyway, the engine now has 169K and uses NO oil. I change it every 3,300 or so and it doesn't even show any loss of oil on the stick during that time.
Maybe one of the pros will comment here. Would replacing only the seals, actually make this much difference?
Because of this engine using no oil, I wonder if the M103 valve seal or guide problem only happens on some cars, or is really prevalent across the breed.
I'll bet that if your car is not showing high oil consumption now, you will probably never see it during the life of the engine.
What do you think pro's?
'84 Euro 240D, 516K miles
'88 300E 5 Speed
Over 800,000 miles in