It appears there is going to be some unusual strategy in the Spanish GP as the medium tyre is 2 seconds slowed than the soft with the soft capable of 25 laps out of 66.
I can see some of those at the back running the medium for a few laps at the start. Those at the front going at least two stints on the soft to begin with the hope of a small number of laps on the medium to finish. If there is a potentially long safety car period then you could change to mediums for the safety car period and change back to soft before the safety car comes back in.
http://classic.autosport.com/news/report.php/id/129501