thanks Bill - i guess the fluid is clear, so water will do. not going to use the truck in the winter anyway, so i can drain it for the off-season.
All you need to do is forget to do that one time and you'll be looking for another jar! If "correctness" is that important to you, I guess that's none of my business. But I would be more inclined to go with something that I knew would not freeze, even if it didn't look quite right.
