White lithium grease (WD-40 brand is ok) on the bolt threads helps reduce the sliding friction on the nut and t-bolt. I only hear the squeak when I get to the tightest point on the turn.
I would suggest far less with every backwash. I have a Pentair QuadDE-100 filter. I use 10lbs on a fresh recharge and never more than 4 pounds on a backwash. That is more than enough DE. You backwash far less than 50% of the DE (especially with only a 90sec backwash), using more than 50% of the fresh charge value will lead to excess DE sitting wasted in the bottom of the filter.