Oil Contamination, Oil Pressure Loss 2.5L V-6

I have a 1999 Mercury Mystique with 125,000 miles on it. A couple of thousand miles ago, the 2.5L V-6 engine developed a clatter at idle (probably caused by stuck automatic cam chain adjuster) followed by the engine oil pressure light coming on at highway speed. I shut down the engine right away and had the car towed to a repair facility. After sitting over a weekend, immediately after the engine was started to pull it into the shop, the oil light went off and the clatter stopped. The oil pressure was subsequently checked and determined to be within normal limits. When the oil pressure sender was removed to install the oil pressure gauge, some very dirty/dark oil came out of the engine. It was decided to flush the engine with new oil and a new filter, running it until it reached nomral operating temperature, then changing the oil and filter again. During this operation the cam chain once clattered momentary and then quieted down. Afterwards, no other abnormal engine noise could be heard. A road test with speeds up to 80 mph was conducted with no problems noted. This was followed by a road trip of about 1,000 miles (mostly at freeway speeds) followed by a couple of hundred miles of local driving.
I had saved the oil filter (ACDelco) that was on the vehicle when the oil light had come on and opened it up for inspection. This revealed at least a tablespoon of brown shellac appearing particles in the remaining oil in the canister and the pleats of the filter were at least partially clogged with the same material. I did not find any metal material of any kind. I also did this on the last new filter (Napa Gold) that was installed after the flush and run for approximately 1200 miles before I changed it and the oil again. There was some of the same particles in the filter pleats and cannister, but not nearly so much as before. I replaced the Napa Gold oil filter with a Pennzoil oil filter and had driven the car approx. 500 mi. When the clatter started again at idle followed by the oil light coming on at hwy speed. I had the oil and filter changed again (Motorcraft oil filter) and after driving for less than 50 miles, same thing happened again, except the oil light went off and clatter stopped when not at idle. The Pennzoil filter that was last removed is partially clogged and there is a small amount of the same brown shellac materiial in the cannister. Any ideas where the material is coming from and what to do about this problem?
(Please note: the quality of the three brands of oil filters mentioned above are of varying quality of construction, the Napa Gold being by far the best of the three [akin to aviation quality ones] followed in order by the ACDelco and Pennzoil ones, both of poor quality, IMO. I have not yet removed and inspected the Motorcraft one.) [Code:1:3d4a0fa42b][/code:1:3d4a0fa42b]