OK, let me just say I'm not an engineer or scientist right up front. I base most of my automotive knowledge on past experience I've had with cars that I've owned. (and I've owned a few). Now I know thanks to computers that we rely on for almost everything these days can safely operate an engine management system. But I think the computers can only be expected to do so much. I mean they can't turn water into wine, or in this case, low octane fuel into high test. There is always going to be that high compressing ratio to consider, and be dealt with. No matter how much the timing is retarded by the computer, the actual space between the piston and the combustion chamber isn't going to change, and the "need" for higher octane still exists. The actual preignition is still occurring to some degree, just not to the point where you can hear it.
It's my understanding, that low octane fuel will ignite quicker, and burn hotter than a higher grade. I think that's one of the reasons it causes the pin holes in the piston and carbon that JMPIQUET spoke about. Eventually, somewhere down the line causing catastrophic engine failure in the form of holes in the pistons.
When a dealer tells you it's OK to use lo octane in your lease car, essentially they are saying, It won't happen to you so don't worry about it. But down the road, after we wholesale your car to another dealer, some poor schmuck is going to have to deal with the eventual problems it will cause. The dealers just look at that as "job security".
I'm not going to tell anybody what to use in their own car, all I can do is recommend something. If you're satisfied with the performance of your car while using lo test gas, then good for you, and God bless ya.
Personally, I have higher expectations from my car, so I feed it the best and expect the best in return. I also didn't lease my car, and assuming my feelings don't change for it, I plan on keeping it for awhile. So I really do not want to be instrumental in it's premature destruction.
I keep pretty good records of my cars, and when it comes time to finally sell it to another buyer, I'll be able to show them receipts for frequent oil changes, and gas records showing I always used the best.
Assuming this web site keeps going, and more threads like this one get posted, I would guess that future "used" Crossfire buyers will check in to see what to look for when buying a previously owned XF, and what to avoid.