If it's syrup, it's fine to decant -- I broke up my first pint into four 4-ounce bottles, originally sold with grape juice (for kids). Clearly relabeled and stored in a home with no children, I might add. The last of that pint was used up about a year after I opened the big bottle, still perfect in every way. The next time I bought the bigger bottle; it's in a pair of 12 ounce glass jars and a couple of those same plastic bottles. I expect it'll be fine for the foreseeable future.
If you're really worried about it, you could get a can of butane lighter fuel and (away from sparks and flames) shoot a little of the fuel into the top of the open bottle just before closing it each time you use some. I don't think it's necessary, but it doesn't seem to hurt.