A 238 (us) gallon ferry tank is a big mother. A bit more than 4 (aussie) 44's.
If oscillation actually occured the back and forth 'wave action' in the bladder tank would have been hard to handle. The trim on the yoke would be fairly slow, I think. So holding the yolk back to stop the oscillation would work, but bad as the aircraft would 'mush' through the air and slow too much. Push forward to re-gain airspeed and the 'wave' comes forward causing the airspeed (and loss of altitude) to be way more than you want,- so heave back on the yolk, and it all happens again!