At around 53000 miles back in January I checked the fluid on my wifes Sienna before a big road trip and the fluid was black. I did 3 drain and refills of the transmission with the reccomended T-IV fluid. Now with about 64000 miles on the van the fluid is almost black again and the transmission is clunking (dead spot)if you let off on the gas pedal and then try to increase speed going about 30 mph in what appears to be fourth gear. This fluid was quoted by Toyota to be a lifetime fluid on the dipstick but I'm wondering if leaving it in there for 53000 miles could have damaged the transmission? We live in a very hilly area of Pennsylvania and the van spends most of its time going from one hill to the next. What is the expected life of the Toyota T-IV fluid?