Re oil pressure... I'm guessing you have a oil tube that is loose. You will sometimes notice a drop in oil pressure if one or more tubes is open. Do you hear any "clacking" noise coming from the engine?
I would suspect the Belt tensioner needs replacing too.
I have a 92 400se...
below is a list of problems posted by W140Gill
I've had most the items on the list fixed too. The engine wiring harness and the AC problems are two $$$ repair items.
I havent' had any transmission problems either.
Rear Headrests not reclining - New switch - DIY
Failed Lumbar Support - Vacuum leak by ZF pump - DIY
Air Con failure - Leak at intake pipe - new intake pipe - Shop
Sunroof Sticking - ongoing - TBA
Rear passenger window sticking - ongoing - TBA
Front door locks sticking - rebuilt front locks twice with new rollers - Removal/fit DIY rebuild Shop
Rear boot lid lock failing - Cleanout and lubricate - DIY
Mirrors not folding - New Switch - Dealer
Rear Centre vents position handles broken - ongoing - TBA
Gear selector light failure - New bulb - DIY
Dashboard lights (fuel gauge/ mileage recorder) - New bulbs - DIY
Dash lights on headlamp on/off cluster - New bulb - DIY
Central lock failure - New ZF Pump - DIY
Water leak - New Water Pump - Shop
Transmission flaring between gears - Transmission Overhaul - Shop
Rear Brake Caliper grinding - New rear caliper + New rear discs (rotors) - DIY
Oil leak at front of Engine - Front gasket of M104 engine - Shop
Squeaking engine drive belt - New belt + tension set - Shop
Drive shaft linkage worn - New linkage fitted - Shop
Ventilation blower motor failure - New motor fitted - Dealer
Ventilation blower squeaking at low speed - ongoing - TBA
Rear speaker buzzing - New rear speakers - DIY
electric aerial stuck down - New aerial - DIY
ABS light on following battery install - Reset ABS light - Dealer
Worn (perished sheathing) ignition leads - New engine wiring harness - Shop
ABS light on following perished ignition leads - Reset ABS light - Shop
Headlamp wiper failure on right lamp - ongoing - TBA
Breakup in lamination on rear glass - ongoing - TBA
Rear screen elements partially working - ongoing -TBA
Centre console roller jammed - forced back into tracks - DIY
Loose Bonnet Star Emblem - New Emblem fitted - DIY
Jump Wiper arm not jumping at screen corners - ongoing - TBA
Pulling to the left - Retracking - shop
Snapped drivers door check-strap (door jam) - New