I"m not sure on proper amount for best performance on this. The way I look at it, SOLID put the fill plug there for a reason. It's kinda like adding a oil pan with more fluid capacity to a motor or a deeper pan to a transmission. They allow more fluid/oil. I'm sure there's someone out there who can explain why but I don't know the answer.

I'll run what level I have in it and report back in a few weeks of driving. I'll be the guinea pig on this. If it fails, then I'm going balls out on a axle swap.