Mobil 1 switched their 5W-30 and 10W-30 oils from a base IV to a base III to compete with other oil companies who got away with calling petroleum base III oils a synthetic.

If you compare the old Mobil 1 30 weight's pour point with the current ones, you'll see that there was a change when they regressed to base III.

Mercedes no longer approve of these two oils.

I believe the new 0W-30 is a true base IV and of course so are 0W-40 and 15W-50.