Repeated oil changes. Change the oil drive 500 miles, change the oil again. The simple act of more frequent oil changes will clean it up better and safer than anything you can dump in the crankcase. After a couple of those keep track of how long it take for the oil to become dirty. If its clean after 500 than go until it gets dirty. Keep that up until your at your regular service interval. You would be surprised how quickly you can clean up an engine.