Almost any form of dust in the ca will work, much like the carbon fill in the or iguana plastic, it gives the screw something more to bite on. The spindles are inexpensive though, so just replace it and then use a plastic/metal thread lock when you install the new one if this is a continual problem for you. I've found they only tend to do this and back out when something else is worn enough that the screw heads can touch the spindle carriers. They shouldn't be able to do that, as the top hat bushings will normally stick out further than the spindle carriers, but if the spindle carriers are worn a lot on the inside they can develop more up and down play with the spindles, allowing the screws to touch and essentially be unscrewed with every turn of the wheel.