Definitely top it off. The fresh dilution will likely be sufficient to offset any ill effects of the older fuel.

You could siffon/pump it out and burn it in your truck, but personally I wouldn't bother. Keep in mind that my opinion is based on running 2 strokers (although my kicker is a 4 stroke). I run mostly wide open and I'm not a troller, so your needs may very well be different than mine.
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