Chris,
I bought 12 gallons when we did our change (per Dave F). Used almost all of it, between the gen set and engine. The radiator and surge tank hold the rest
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I had a hard time with the funnel I had filling the radiator, so I took some PVC pipe with 90's and 45's and cut to fit to ease pouring the coolant in. Maybe you won't need to do that with the ISL.