Which oil...Castrol Blend, HM or Mobil-1?

[DSteven]

My parents have a Ford Explorer 4.0L SOHC and a Taurus 3.0L SOHC. Explorer gets 1-3 mile trips every day with 100+ mile trips on weekends, about 15,000 m/year. The Taurus gets 1-3 mile trips every day with 80+ mile trips every now and then, about 9,000 m/year. They were getting Mobil-1 synthetic 5W-30 for 10,000 OCI, with a new filter and qt at 5000. They were switched to Castrol High Mileage and Syntec Blend oils Dec '03. This way they get OC's every 2500-3000 miles. He is notorious for taking good care of his vehicles, but running them to 200,000+ miles. Which combination should provide for the best engine protection and longevity? He is set on the OCI's with each oil. Both engines operate great on either oil. The best MPG is on Mobil-1, then Castrol Blend, and then the HM. What are the opinions of the wise? Thanks.