Mercedes Benz repair: vac system and LT engine maintenance, synthetic motor oil, vacuum motor

Question
My 1983 MB 300D Turbo 321K runs very well.  I am having a problem with the vaccuum system.  It seems to loose pressure after an hour or so for the doors and maybe 4  cycles of the locks, additionally the center air ducts are "stuck" in the closed position.  They have been closed this way for 6months or more and have opened randomly 2 times, but since closed back after shutoff.  Also the passenger door lock was unlocked while the vaccuum was locking them and the key entry for the passenger side door lock does not work now.  Any suggestions for these issues.  Also, I have been told to clean away sludge and buildup inside the engine to drain the oil, fill with diesel and run for a couple minutes then replace.  If this is not OK should I even worry about buildup inside the enginge?  There seems to be very mild power loss over the past 1-2 years.  Thanks.

Answer
A few issues.
NO, never run diesel fuel in place of engine oil, read the following thread for further information.
http://www.mercedesshop.com/shopforum/showthread.php3?t=101404&page=1&pp=15

The vaccuum system leak down problem = cracked line, hose or vacuum reservoir bottle.
Replace all rubber hose union sections through the entire car, a bulk roll of vacuum hose and wire cutters + a lot of patience will be needed.

The center air ducts will be:
#1. Cracked or disconnected vacuum hose.
#2. Bad vacuum motor = vacuum diaphram.
#3. Corroded and plugged in line vacuum restrictors.

The passenger door lock that was unlocked while the vaccuum was locking them = jammed lock and linkage.
Remove the door panel, lubricate the key lock and linkage,  move them by hand until they free up.

The key entry for the passenger side door lock does not work due to jammed linkage.

Use full synthetic motor oil, it will clean the engine out in a couple of oil changes, no more is needed IMO.

Mild power loss over the past 1-2 years:
Could be plugged fuel filters, ALDA pressure feed or the crossover valve, read this thread:
http://www.mercedesshop.com/shopforum/showthread.php3?p=733935&posted=1#post7339...