I've been using Amsoil for about a year now with no adverse affects. I've found that my cars temp does not fluctuate as much, runs slightly smoother and definitely starts up better. I change my oil every 1500 miles/6 months so I'm not sure about the longevity. I've read that time is just as bad as mileage so it's best to keep short intervals if you don't drive the car frequently.
I pay about $55 (Including shipping) for a case (12 quarts) of 10W40 Synthetic.
02 C32 AMG
84 190E 2.3 5 Spd