I had Mobil 1 10w 30 synthetic in the tranny for a year. I drained it today, it came out clear as new. A good thing. I always thought the shifting/ syncro performance had not been equal to the fluid that was in the car before I changed it. The "too slippery theory". This proves it. Conventional oil in MY tranny (which has ~120K on it) (knock on wood) shifts better.
For the oil change, I switched to Valvoline Max Life from Mobil 1.
K&N oil filter and 4 quarts = 18 bucks
That beats syn-pathetic at 30 bucks anyday.
Feel free to discuss.