I will preface this by saying I am a complete idiot.

I have a 1985 300CD that, after an odd turn of events, turned from a superb runner to an asthmatic dog. The last time it was running great was yesterday, after I successfully finished a valve adjustment. I did the procedure by the book and it turned out great. So smooth! I took it for a nice long test drive to enjoy the fruits of my labor.

Later that day, I was headed out to visit some friends about 2.5 hours away. The 300CD had been on the highway before, and after the valve adjustment, I was feeling confident about the performance I could expect. While still on the city roads, I noticed that if I braked, the engine would shudder slowly from left to right. I remembered this phenomenon happening just after I bought the car last month, and it went away after I did an oil and filter change. Figuring this could be cured before I left on my trip, I pulled into AutoZone to check my oil level. This is where it begins:

Pit-stop 1: Stopped at Autozone and checked my oil level, which was about halfway between the full and low marks. Went in to get some Delo to top it off. I was careful not to put too much in, checking the oil level as I went. Job done, I cleaned up and went on my way. Three miles or so down the road, I know something's not right. The engine is rocking a bit more at idle, so I pull over at the next gas station to see what's up.

Pit-stop 2: I check my oil level again, and I feel like an idiot. The level is about a quarter of an inch (maybe a bit less) above the full mark. I decide it's not a good idea to keep on driving like this, so I pull out my 13mm wrench and oil drip pan and figure I'll drain the excess and fill up as needed. This doesn't go exactly to plan, as the drain plug is pretty hard to replace with hot oil gushing out. I end up draining more than I wanted to, but I don't panic because it's in the drip pan. Eventually I replace the drain plug, pull my drip pan from under the car and refill to the precise level, waiting and checking several times to make sure it's right this time. I let the engine idle for a few minutes and resume my journey. The shaking I experienced at overfull is now gone.

Before long, I pull on to the highway, and it doesn't take long for me to know there's a problem. The engine is knocking and making a light, airy whirring noise. Power is way down, and I need to really put the pedal down just to go 50 mph. I pull over at the next offramp and see what's going on.

Pit-stop 3: I check the oil level, which is still perfect, exactly at full. I check closely for leaks; there are none. I don't see anything that's obviously wrong, and the engine runs fine unless I'm accelerating or at highway speed. I decide to pack it in and drive the merc carefully home.

There are a few possibilities that have crossed my mind:

Oil contamination: My drip pan is a sealed one with the lid that screws on top. I bought it brand new, and used it previously to drain a lawn mower and a Toyota Corolla, and I disposed of the oil afterwards. The last time I used it was for my last 300CD oil change; the oil was drained while hot and discarded before it had a chance to cool, theoretically dissolving much of the reside from the other oils that might be there. Thinner oils for a diesel are not good, but if there's any in there, it's in trace amounts.

Metal contamination: In theory, metal shavings from the bottom of the oil pan might more easily find their way into critical components when the same oil is re-filled. But really? The OM617 has a (new) oil filter that should take care of that

Mechanical damage: Either from excess oil or from the shaking it caused

Fouled injectors and/or glow plugs: Excess oil might have fouled these, leading to incomplete combustion, causing the power decrease and strange noise

Sorry for the long read, but I can't quite wrap my head around this one. It pains me to consider things like scored piston rings or valve damage, but either way, I love this 300CD and I'm not giving up on it.