The only oils I've ever seen with deposits on the bottom are always dinos. The only synth I ever saw deposits in was Mobil 1 75W90 GL5 gear oil. It had a crust on the bottom that wouldn't break up and mix in. Synth motor oils though,I've never seen any deposits whatsoever. I have a 6-8 year old half quart of Amsoil AGL that sat undisturbed all those years,and when I recently found it,there were no deposits at all,still looked perfect.