Will some engines get more mileage from lower octane fuels rather than the higher octane fuels ?

For the most part it won't make a difference as long as the car is running at normal compression ratios. When you experience pinging in the engine because of poor fuel or to low of octane then yes, you will loose mileage. Pre detonation is not just harming your valves, it's letting pressure escape from your cylinder at the wrong time, so instead of the energy of the fuel combusting being used to propel the vehicle, it is being wasted. If this is not happening, then you are gaining nothing with higher or lower octane. One thing I do have to mention is not all gasoline is created equal. Most of all the refineries pretty much make gasoline the same way, they just have different additive packages. There is also blended gasoline which is made of different components from various petrochem processes. These side streams are collected, then mixed together in proper ratios to meet gasoline specifications. I wouldn't say these are all the same. Some small companies out there that do this put some real garbage in there. Another point that will make a bigger difference than octane is the fuel being moderately fresh. If you buy fuel in the summer that was left over from the winter because the gas station is in the middle of no where, you may run into issues with vapor locking in your car. The winter grades gasolines have a higher vapor pressure and are more volatile, so may turn to a vapor in you fuel system before they are supposed to in the summer. The opposite applies as well. The lower vapor summer fuel may not vaporize when it needs to in the winter leading to poor combustion.