I don't tend to have leftover guacamole, but I have not found much alters the course of oxidation of avocado halves. My best trick is to toss them in a ziplock bag (not because it helps, just to keep my fridge tidy), keep no more than a single day, and use a spoon to scrape off the surface browning before use on day 2. I lose about a tsp in total and am hard pressed to tell the difference on what is left, and it's easier and neater than oiling, lime juicing, or anything else.
_________________ Carey
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