RE: Futaba RX's & Servos: NO Lithium Bats, 6v Max??
You are spitting into the wind with your Lithium argument.
Futaba recommends 4.8v or 6.0v maximum to retain warranty. Two cell Lithiums are higher than that and Futaba usually states that if you want to use them they must be regulated.
Servos are designed at various voltages for optimum operation. Go beyond that and its service life can end abruptly.
Freshly charged A123's and LiFe packs taper to 6.0v very quickly so can be used providing the servo is also rated for 6.0 volts.
There is no mystery as Futaba's voltage specifications are on the individual products web page.
A lot of radio equipment gets smoked because guys don't pay attention to the specs or think hey know more than the product engineers.