I'm the guy that keeps his cars until the wheels fall off. When Syntec came out, I swapped it into my 89 Corolla instead of the GTX I had used faithfully before. That was at about 95,000 miles. Then Sam's started carrying Mobil 1 at about 240,000 miles so I switched to 15W50. Similar story on the 4Runner only it has 175,000. Based upon your knowledge what would you recommend....and why? I'm a Chem Teacher in El Paso, TX, with no experience in petroleum, so I'd like to hear your justifications. Thanks!