we use B&G En-force which is similar to PB blaster but seems to work a bit faster. along with a fuel induction/injection flush works great to help you get them plugs out. the last one I had was @ 120k and all 8 still broke off with less than half of a turn of the plug with literally the force of a baby turning them. Engine was completely sludged up along with carbon buildup on not only the heads and plugs but the runners were froze. I thought the customer would need a new intake but that En-force cleaned it up really well. The best tool for one of these broken plugs is from Cornwell. It takes about 1/2 hour per plug if you do need to remove one.