I asked some folks I know about blend-at-the-pump gasolines. RandyO mentioned that there was a time when one company offered numerous octane blends by blending at the pump. After rolling on the floor laughing for a while, the chemists said that, yes, they remembered that and, yes, it's possible to do it, but practically speaking it doesn't work. The explanation was a bit above my head, but the concensus was that you could probably blend regular and premium, and if you really watched the hydrocarbon population of both grades, come reasonably close to an intermediate octane. There were numerous factors that would affect the success of the procedure. It would be harder to do it right in hot weather because of lighter hydrocarbon flashing (the gasoline in the storage tanks changes slightly if it sits there a while because the lighter ends evaporate). The gas you actually pump into your tank may or may not be the advertised octane depending on how fresh it is, hence the recommendation to buy gas where the station sells a lot of it. There were a lot of other caveats about the procedure, but in a nutshell that's it.