Synthetic sounds good. I recall talk that it helps dissolve gunk. Synthetic diesel oil is even better since CJ oils have more detergents. Some talk of using ATF since it is oil with even more detergents (I think), but you are going "off manual". Decades ago, auto parts used to sell quart cans of "engine de-gunker", which was more like diesel fuel I think. You idled with it for 5 min, then drained the pan. Might have done more bad than good. One thing I would try is drain the pan, remove the valve cover and brush down the valve train w/ diesel, or even better bio-diesel or even corn oil. Continue until you fill the pan and let it sit for days to degunk the oil pickup screen, then drain, pour down some clean oil to sweep gunk out, then change oil every 500 miles until it looks like normal diesel - dark but not gunky.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans