That engine doesn't require any sort of *special* coolant, regular coolant in a 1 gallon jug (50/50 or concentrate is again your choice, I never use 50/50), whether or not you prefer long-life over normal is your choice (just don't mix them as that defeats the long-life properties).
But do NOT use tap water to add to the concentrate, use deionized or at the very least purified water. Tap water is not softened, with time it will develop lime deposits in the system.
*One other note*
the manifold bolts are torqued to only 18 FT.LBS, an inch lb. torque wrench is far more accurate, most 1/2 drive torque wrenches don't begin to read until 20 ft.lbs.
18 ft.lbs is 215 inch.lbs