It is shocking to find out that only now do we know if old mayonnaise is an effective thermal compound or if, like on a sandwich, fresher is better.  Thanks to Hardware Secrets the mystery has been solved by a 12 day test which proves that old mayo is every bit as good as fresh mayo; not as good as real thermal paste but certainly better than nothing.  Strangely there is no mention if the scent of matured warm mayo improves your computing experience, obviously more research is required.

"Back in 2011, we tested mayonnaise as a thermal interface between the CPU and the base of the cooler. Amazingly, it worked nicely, proving to be, at least in a short term, a better thermal compound than several well-known commercial products. But, at the time, we could not determine how long it would continue to work before a degradation in performance was noted. This time around, we ran a long-term test to determine if mayonnaise can last as a thermal compound."

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