I have a 2002 dts 4.6 northstar and I am going to be scheduling to have the dexcool changed in the next week or two. Before I do i need to know the best prefered method? I have called about 5 to 6 cadillac dealerships on how they mix the dexcool and have gotten 3 different responces. some of them said they mix it with tapp water (which I believe should be the wrong way), a couple more said they mix with distilled water and a few more said they get a premix already mixed from gm.... which way would be the prefered method here???