Proper coolant in the 6.4 powerstroke is very important. Ford premium gold coolant is what is required. This coolant is different from the conventional green coolant in the way that the properties of the coolant are made. One other thing that I have been seeing lately is that people are putting this stop leak in there system which will cause you to have repairs. It is very important not to do this, that stuff will clog up the oil cooler and cost you about $1700 in repairs. If you have a leak add coolant (premium gold) or water until you can get it fixed do not put stop leak in your system save yourself the repair cost.