Great info seems to be consistent around the room, I'll start with the easy stuff valve covers to get a better idea, should I try a top engine cleaner vs the diesel thing?
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I have have heard it said that the old farmers used to run kerosene through an engine to clean it. Kerosene is similar to diesel.
I used to add a quart of transmission fluid in my oil before changing it. Transmission fluid is a good cleaner, and still an oil. I would let the engine idle, let it get warmed up, then drain. It will thin the oil, so I was told to just let it idle rather than rev it up. Change the oil & filter. Did this on several "old" cars I had and never had a problem.
So you might want to pour several quarts into the oil pan with the oil pan plug installed and let it sit for a week and let the trans fluid soften and cut through any crap on the bottom of the oil pan. Then drain it to flush. I used the Dextron trans fluid, but I suspect the diesel or kerosene would do the same thing.
Then you might add a new oil filter, your good oil, 1/2 quart Dextron to that and fire it up and let it idle and get warmed up. Then shut it down, drain the hot oil, install a new filter, and fresh oil. Gonna cost you a little, but it may do the job.
No guarantees, as you still may have a bad head gasket or something else gone wrong, but it could be worth a try if nothing else to lose.