How often should oil be changed ?
That depends on the brand and type of synthetic you use, as well as the type of oil filter. Mobil 1 makes an extended service full synthetic, which they claim is good for 15,000 miles between changes. Amsoil full synthetic advertises 25,000 miles between changes; but only if you also install and use one of their super fine bypass oil filters, in addition to your car's regular oil filter.
Since the actual mileage you can go between oil changes is very dependent of the type of driving you do, the brand of oil you use, and the condition of your engine; there really is no hard and fast rule. That's why commercial vehicle fleets regularly take samples of their oil, and send it to laboratories for analysis to determine whether it needs changing. Sometimes it is necessary to change oil more often than expected; other times you can go longer between changes. My experience has been that, when oil is nearing the mileage where it needs changing; the rate of consumption will significantly increase. So when my car needs a quart added, it usually means it ought to be changed.
Regardless of the oil you use; if the change intervals are longer than the car manufacturers recommendations, it will void your factory engine warranty. That said; I have typically felt comfortable with 7,500 mile change intervals when using full synthetic oil of an appropriate viscosity.
I would also add that it is much safer to stay with only one brand and type of oil for the life of your vehicle; this is true for both conventional and synthetic oils. I have seen too many engines ruined by switching to a different brand or type of oil. Mobil 1 is chemically different from all the other synthetic oils, and is closer to petroleum oil in its characteristics. I prefer it for that reason. They also make a european formula 0W-40 full synthetic, which is available in the U.S, and is my preferred viscosity for all around use. It is not one of their extended service oils; but the protection is far superior to other oil grades. Not all stores carry it; but it can often be ordered from stores which carry other Mobil 1 products; and can always be bought from the local Mobil oil distributor.
