Before you go tearing things apart try using something like Marvel Mystery Oil in the crankcase and in the diesel. Read the label. On the crankcase I'd put in the amount per number of quarts of oil recommended on the label. On the fuel additive I'd run 2x-3x what it says. Thinning the oil too much might cause other problems but in the fuel it will burn.......
Run it awhile at fast idle (to keep the heat up but minimize wear) and turn it off and let it sit for an hour or so.....give the stuff time to soak in. Do that several times.
If you get it cleared up I'd add something to the diesel for upper cylinder lube. I used to use a quart of ATF to 100 gal of diesel on my OTR and had zero problems. Cheap insurance.