... 400 year old oak ... as you would expect very dry.
I just don't go with this. Oak and any other timber put into any environment will adjust its moisture content to match that of its surroundings. Most modern homes have a moisture content of 10% to 12%, and it doesn't matter if you keep wood in them for 400 hours or 400 years, it will be the same moisture content as the room that it sits in. As Steve (Seventhdevil) said, get your tool technique right. Use a shearing cut and if your tool-handling skills are not yet up to it, then practice, practice, practice. Washing up liquid is not a short cut! Soaking seasoned timber in water for several weeks is just about the weirdest thing I've heard of.
One other thing to consider......all trees absorb minerals out of the ground through the water that they capiliarise. When timber is processed and dried, those mineral-rich fluids start to dry-out and eventually the minerals begin to solidify, or even to calcify. In many respects, this is the very earliest stages of fossilisation and it is why very old timber is sometimes said to be like stone. Washing up liquid does not reverse this process, but whereas several weeks of saturation will soften the remaining, uncalcified wood fibres, it will also do untold damage through the uncontrolled expansion and subsequent contraction of the wood.
Forget Fairy Liquid....work on your tool technique. Seek out a fellow turner who knows how to perform a shearing cut and learn from that turner.
Les Symonds.