The MD80 autopilot used to allow altitude fluctuations of up to 300 feet plus or minus from selected altitude in cruise.
This was not only a turbulence mitigation feature but also a fuel saving concept, if speed was a little low the autopilot would drop the nose slightly allowing speed to increase without moving the autothrottles 'sacrificing' a little altitude to achieve this.
Conversely, with updaft turbulence the autopilot allowed a positive excursion of up to 300 feet.
Of course, this was all in the days before RVSM, can't allow this anymore
