My 2 cents.. Get some cheap dino oil. Bulk stuff. Run short OCIs. Just visually inspect and change when it's dark. I had an old VW that was sludged up when I got it. There for a while I was changing the oil every 200-300 miles. Each time it would come out looking like diesel LOL.
Careful with the fast clean stuff. If it can't dissolve everything you'll end up with sludge bits breaking off and either clogging the passage ways or clogging the screen. Gentle is the best approach. Leave the cleaning/flush stuff to when you're sure you can't get any more out with frequent changes