What happens when you are low on the fluid in the reservoir? Does the back sag? Does it loose it's ability to recover quickly after a bump?

The SLS system only plays a role when the rear is loaded down beyond the normal ride height. Otherwise, the valve is mostly open to return the fluid to the tank. Dampening is like any other gas shock only in this case the gas is in the spheres.
If you are low enough on fluid for the pump to cavitate then I suspect it may feel as if the rear shocks are not providing dampening.
If the rear sags with no load then the springs are weak. The sls is not intended to raise the normal ride height, only assist with added weight.

@Whiskeydan - That was a very detailed explanation thank you, I had 7 people in the car the other day (before I realized I was 1 & 1/2 qt. low on fluid) and the bag was sagging, but I wonder how much better the assist would have helped had the fluid been up to par.

Is Shell Tellus 22 suitable for self leveling SLS system?

Was told that Tellus 22 replaces Tellus 15 and 17.

NPK