Putting anything lower than the recommended octane rating will obviously cause knocking/pinging. Which will eventually cause all sorts of engine, chamber, spark plug problems. All that headache for an extra 2-3 dollars on a fill up? Not worth it.. if you have to think about saving money $2 - $3 dollars on a fill up then you might as well sell the car and save yourself the money in repairs because of that train of thought.

I say never put in 87 in your XF; ever. Unless you're outta gas in the middle of nowhere. Even then, I'd only put in enough to reach the nearest 91 octane station. Also, always pump the gas yourself and if you have to do full service make sure you make it clear. 91 octane or higher only.

I remember being out in a convoy with a couple of buddies of mine and we pulled our humvees into a Shell.. Went inside to grab some snacks and soda and stuff and come back to find the attendant had filled some of our humvees with gasoline. Ever since that day; I pump myself.