DON'T LUBRICATE THE PISTONS!
You'll contaminate the pads and render your brakes useless!
The DOT or Mineral Oil does in fact lubricate as it works.
With many disc brakes, only one side moves anyways. The outside piston is the one that moves, The inside one is fixed and the outside piston pushes the rotor in toward the fixed one.
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"Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming, "WOW, What a Ride!" - unknown
"Your Bike Sucks" - Sky Yaeger