Get a flush kit, that comes with chemicals. (usually, its something like muriatic acid.) Do a chemical flush on it, or, take it to a shop with a flush machine. They will have even better chemicals than you can get over the counter.

 

Scoop on the end is it took over a month to get the thick tar oil flushed and ready for hoat coolant. What a mess that stuff made all over and it took 30 def jugs of water and tar a day for a month. I drained it,filled it with water and dishwashing detergent,would run it for the day,then drain the next day and fill n flush all day. Over a month to clean it up. It's running perfect now. It cost $2000 for a new oil cooler and I got a deal online for one,the shop was going at $180 an hour,this job would have been at least 80 hours in a shop,it took me way more hours. Not a fun ride but got it fixed.