I've personally tested many types of oil while in the military. Synthetics are far superior in every way. I've ran Amsoil 5w-30 in my 86 trooper tranny for 336,000 without a single problem. Own a large bass boat and towed many trailers all over Arizona,(just about the hottest spot in the nation), and California. I did tear down my engine at 206,000 due to catalytic converter clogging and cracked the head, but the bottom half was not just within factory specs, but on the minimum side! The inside of the engine was totally clean. It's one thing to always read about and do tests on the stuff, it's another to actually tear down your own engine to find it almost brand new 8 years later. I could personally run lab test of synthetic, specifically Amsoil and some Castrol syntecs, that had 25,000 miles or so and it would still outdo brand new conventional oil in just about every catagory, (falex wear tests, boil off tests, etc). Synthetic gear oils (normally Valvoline was tested) give approx. double the protection while the Amsoil I tested one time gave approx. 3-4 times the gear protection while still maintaining a positive flow at -30degrees plus! While conventional engine oils have improved in high heat environments, conventional gear oils cannot hold a candle to synthetics.
Not puttin down conventional lubricants. They will protect your vehicle just fine, especially today's more advanced conventional lubricants. But for the long term performance minded person who demands only the best protection and longest life of their internal parts, synthetic lubricants must be used. If you cannot find Amsoil, Castrol makes a very good synthetic and Valvoline would be 3rd.