Then you shouldn't be driving it. If you have milkey oil then the head gasket would be a good place to start. Hopefully that is what it is and not a cracked block or something. Cylinder leak down tests are more for valve and ring diagonisis. Now if you do a pressure leak down test on the cooling system, that would make more sense. Personally, I think you need to rip the head off and have a look see. A better test would be to drain some of the oil into a clear container like a glass or jar and set it aside for a while. It's best to do this from the oil drain and after the truck has sat for awhile as water will settle in the bottom of the oil pan. If there is coolant in it, it should seperate out soon enough. You could get the oil analysed too. That shouldn't cost as much. If you've got milkey looking oil, again you should not be driving it or you're gonna toast all the bearings. When that happens you'll be engine shopping or another overhaul. For your sake I hope it boils down to only a head gasket. Considering the other possibilities, that would be the lesser of the evils.