I know this is old, but I bought a plane that has 3 of these old FP-S128 servos and was wondering whether they could run on 6.0 volts. The fellow I bought it from had just recently installed a UBEC and set it at 6 volts. They seem to be working fine.
I found an instruction manual online for the Futaba FP-2GS transmitter which comes with two of these servos. See photo below.
The servo specs say the power requirements are 4.8V - 6V. Since it mentions current drain at 6.0V, I assume the torque of 48.7 oz-in and the speed of .24 sec/60 degrees are also at 6.0V, although it does not specifically state that.
