Back in the day we used to put a quart of diesel in the crankcase before draining the old oil, let it run for a few min then drop the oil.
Today, there are so many things that you can add to the oil that it is up to you to decide. Any additive that is designed to flush the engine of sludge should help the situation, just follow the directions exactly as directed.
Once done, use a good quality oil and change regularly.
I don't have a sludge problem, but the ZJ sat for a while so I am flushing/conditioning mine at the moment with Marvel Mystery Oil, before I drop the oil again. I use Mobil 10W-30 conventional.
1998 ZJ Limited 5.2L (upgrades in progress)