BGA package, should I add some solder paste?
You can get small bottles of liquid flux suspension that don't make a mess of the array, the flux is usually suspended in isopropyl, additional benefits are that you can use a syringe with attached needle to add flux and reheat if all the balls didn't connect properly during initial heat-up, and  you can usually just apply the stuff and heat the chip without having to change the balls.

The greasy stuff can sometimes join the balls together and can short the chip or board.

Tip: I noticed a guy put a (not too heavy) small weight on the chip after heating and it helped the joints to cool down whilst properly joined.