If the motor has no leaks, there's no reason under the Sun you can't switch over to either type of oil. You're really ahead of the game with 5W-30 100% full synthetic Mobil1 from Walmart in a 5 quart plastic jug. Oil companies never advertise the exact percentage of synthetic oil in their "synthetic blends". Walmart also has Motorcraft oil filters which are better than anything else on the shelf.
Go for the gold right off the bat with full synthetic.