Two different vehicles have joined my family recently and I was wondering what you guys thought about the repurcussions of switching them to Mobil 1 full synthetic.

2000 Ford Explorer, with a straight 6 cyl, 64,000 miles
2000 Ford Focus, Zetec 4-cyl, 56,000 miles

Do you guys think this is a safe switch for either, both, neither?

Thanks very much.
-Dylan