If you know it was changed to Mobil1, then it is synthetic, and I'd recommend sticking with the Mobil1 definitely. I don't recommend doing an "overkill" and using Mobil1 combined with 3000 mile or so oil changes. I'd be happy using the Mobil1 combined with oil changes out to about 8 to 10000 miles or so. Be sure to check the oil often, however it takes a long time for the oil to fill the oil pan after shutoff, usually checking it after filling up the fuel tank for example is too soon to check it, although it's good to check it hot. I'd recommend checking it about 20 to 30 minutes after shutdown after a trip in which the engine reaches normal operating temp, this is how to get an accurate reading.
I'd also point out that if you are in a region of the country where the weather turns cold in the winter (cold enough to create ice, at least at night), when removing the oil cap in cold weather (below 40F or so ) you will notice a whitish-yellowish "paste" under the oil cap, this is normal. It is only on the cap and possibly the dipstick, it's not in the engine internal parts.
I'd normally recommend 0W40 Mobil1 if you can find it, other viscosities will work OK if you can't find the 0W-40. Usually the dealer will have the 0W-40, but it's not important enough to buy it from the dealer, they'll charge an arm and a leg for it and it's not all that important. If your in a cold climate I'd use 10w-40, if your down south or in a summer climate, 20W50 would be good, just make sure the viscosity you are using is appropriate for the forcasted weather, in other words get the 20W50 out of there before Christmas .

Gilly