As most of us know, the drivers are only allowed the use of 8 engines per season. It is a 19 race season, so that means that each engine will have been used, on average, 2.375 times. To be less precise, one must use 5 engines 2 times, and 3 engines 3 times. Of course, it never works out that way. There are failures to consider, as well as other unforseen circumstances. The teams often use high mileage engines for practice and then change them out for fresher engines for quali and the race. There is no penalty for this strategy.
The teams are also allowed to change engines one time after quali without penalty, but are restricted to using that engine from that point on for practice only. However, you are allowed to use that engine freely in the final race. This is what I think Ferrari have done. Look at the following engine use graphic. This graphic was supplied by http://www.f1wolf.com/ and the exact page is http://www.f1wolf.com/2010-season.
Alonso used engine one for practice and quali 1 time. My conclusion would be that his #1 engine is the lowest mileage engine in the field. If it was not heat stressed in practice and quali at Bahrain, it will be the freshest engine in the field.
Thanks to P-One and others at http://the-pit-lane.co.uk/index.php? for helping me to inform this blog via discussion on their forum.