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Its not solvent stuff, its Valvoline Premium Blue "RESTORE" engine oil, developed by Valvoline for Cummins for a piston ring sticking problem they have.It's possible the rings are just gummed up. That solvent stuff you had to the oil makes me nervous though. I've seen some LS oil pumps suddenly lose pressure.... And found little tiny pieces of grit in the pressure relief valve. It would also make me nervous if any little pieces made their way to a lifter. The filter should catch it, but then again on a cold start with high oil pressure, some oil bypasses the filter through the oil filter pressure relief valve above the oil filter on some, and built into the oil filter on others. I can't remember what you said your mileage was. If it's around 200,000 miles, it's probably a little bit of everything. (Rings, valve guides and valve seals)
I feel your pain, mine's doing it on the drivers sideSpent some more time under the truck today and noticed the passenger side rack and pinion boot is wet this could be what's causing the front cover to be oily