If I understand
their warranty correctly, one of the advantages of Mobil Clean 5000 is that, so long as your engine is in good working order (presumably you would have documentation indicating previous maintenance) when you put the oil in, ExxonMobil will warrant that your engine will not suffer a lubricant-related failure due to using Mobil Clean 5000 for 5,000 miles/6 months.
The warranty is renewed when you change the oil, even if you previously went over the 5,000 mile limit (it'd obviously be void for the time you're over 5,000 miles).
Pennzoil/Quaker State/Valvoline oils only seem to offer their warranty if you use their oil's for a long time (I don't recall exactly, but I think Quaker State wanted you to use their oils from the first oil change and change every 4,000), and the warranty was void forever if you went over 4,000 miles, even by a single mile.
I'm sure that ExxonMobil, like any other major company, would try to wiggle out of any warranty claims, but it's still comforting that they're willing to warrant their products in such a way.
My mechanic uses Mobil Drive Clean (as mentioned before, a notch below Clean 5000) and a used oil analysis for my car looks pretty good.