I agree with Blisk. The water you have in your trunk should be ok, I would jus rotate it. having lived in the midwest (Oklahoma/Kansas) I know there is a lot of water available if your waer goes bad. I think that if you rotated your water once a month or two, you should be OK. Since it sounds like you dont have too much, maybe just rotate everytime you change your oil!

Garrett
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On occasion of every accident that befalls you, remember to turn to yourself and inquire what power you have for turning it to use. - Epictetus