It is possible that a piece of cloth or a piece of gasket material or a piece of silicone has clogged your oil pump at the oil pump screen. I'm not certain about on the 350 SDL, but on the 300 SDL, the oil pan has a side entry into the oil pan whereby the bottom of the oil pan and the oil pump can be accessed although it is only about a quarter of inch from the bottom of the oil pan to the screen's base in the botton of the oil pan, but there is sufficient room to remove an obstructive object. Drain the oil, remove the side entry pan on the main oil pan, then fabricate an "L" shaped gadget (clothes hanger or something like that) to scrape without using excessive force, the oil pump screen and see if anything is blocking the oil entering the engine. The best method of checking this would be to remove the oil pan, but that is a major job, but is the only way to be certain that there is no blockage. It has happened to me and that is how I know that a small piece of gasket material, like the kind on a head gasket, will obstruct passage of the oil getting into the engine. The oil pressure would read low and your lifters will make a tapping sound because of low oil pressure. And you did say this came about after an oil change, which disturbed the contents at the bottom of the oil pan. On the other hand if the lifters made their normal sounds and the guage read low, then it could and more probably would be the guage and not the oil pump. But with the low pressure, and tapping lifters, the guage is warning you properly, more likely than not, since not enough oil pressure is being built up to keep the lifters from making the tapping noise. The oil pump is chain driven, which moves a lot of oil when the screen is clear of any obstructions and makes it less suspect although anything mechanical can and will fail. Also, it is near impossible to see the actual screen, unless the oil pump is removed, but an "L" shaped object should be able to detect a piece of gasket blocking your oil pump. Maybe even a mirror and and light, which I didn't try. It would also be a good time to clean the sludge out of the botton of your oil pan, which come to think about it, could also be part of your problem, if the sludge builds up high enough in the bottom of your oil pan to restrict oil flow to the oil pick up, which is protected with the screen to keep large objects from entering the engine and doing damage. I hope this helps.

BenzDiesel

the o rings being hard should not have caused a problem all at once. the invoice said oil bulk, so that means that they took it out of a hose. so if the wrong hose was pulled down you would get the wrong oil, even possibly tranny oil or such. i am thinking wrong oil. i would drain it out or at least a quart and look at it. if any suspicion of it i would change it pronto. i am thinking looking for a mechanical problem is barking up wrong tree.

__________________
[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

easyrider,the 91 3.5 dosent have any access to the oil pump except removing the oil pan and that is something that should be completed on the 603.970 engine in my opinion.if you want to read there is a lot of earlier comments about these engines.i can tell you for sure that these engines are a ticking time bomb,bolts come loose from the oil baffel plates and get into the oil pump and it will destroy the engine.it is possible that the washers have already reached your oil pump screen and have created restriction,but i cant see the pump going to zero with just a couple washers there now.
one suggestion for low oil pressure would be a leaking oil sensor which is located at the oil filter housing,but you should be seeing the lost oil on the ground as it leaks from the belly pan. this should not cause any additional noise fron the head unless it is a major leak and you should have seen the oil down on the dipstick after a 350 mile trip.
larry perkins
72 old cars

just wanted to know if there was any feedback on this problem ,please post if so.
larry perkins
72 old cars

Did you replace the oil filter? I would suspect a filter problem if you suddenly have low pressure, as there is an internal seal that will dump the pressure feed back into the pan through the bypass filter return if bad. No reason to expect that an occasional oil filter is defective, especially today. Get a Wix, Mann, or Knecht, avoid Bosch as they are now made in India with very erratic (if any) QC.

Your oil pump cannot be starved of oil unless there is none in the crankcase or the pickup tube is shot, it's not oiled by the pressure main.

Main bearings, on the other hand, can be oil starved and badly worn, causing low oil pressure. They will knock badly then, too, but you wont' mistake that for noisy lifters!

Peter

__________________
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!

Did you ever find and answer?, My wifes car has the same problem.

In my case it was a loose wire on the sender.