Sludge is usually caused by pellet type salt. Dirt in a brine tank is not the problem people think it is but yes, some salt is 'dirtier' than others.
All softeners can use any type of salt (sodium chloride) and/or salt substitute potassium chloride. Depending on the salt efficiency setting, you may have to increase the salt dose by up to 27% to use potassium chloride. It also may cost 4 times more for the same size bag of sodium chloride.
Salt consumption is based on the programming of the control valve or height setting of a float in the brine tank. Either way it is set to the actual number of lbs of salt to be used per regeneration by controlling the amount of water added to the brine tank during brine refill to dissolve 3 lbs per gallon; it's actually temperature dependent but 2.7 lbs per gallon.
This is based on 21 years of study, selling and servicing all the different types of softeners. All softeners have an adjustable salt dose in lbs and thereby an adjustable K of capacity.
The salt dose lbs used is dictated by the cuft volume and type of resin in the softener. The volume of resin dictates the size of the resin tank. The size of the resin tank dictates the control valve that can be used to service that size tank.
Here's how it works, two identical softeners, lets say 1.0 cuft (30k max) of regular mesh resin. One is set at 15 lbs and that regenerates 30K, the other is set at 6 lbs and gets 20K. The one using 6 lbs has a salt efficiency of 3333 grains/lb. The one at 15 lbs and 30 K gets 2000 grains/lb. You do that just like figuring gas mileage.
That applies to every softener in the world, regardless of the brand of the resin, brand of control valve or the brand name on the softener.
The number of regenerations can be important for both water and salt efficiency. Example, Kinetico says one of their softeners uses only 2.2 lbs per regeneration and gets like 5000 grains/lb. and uses only 11 gals of water per regeneration. They always stop there.
What they don't tell you is that it may have to regenerate 1-3 times per day while they compare 2.2 lbs to a softener using 6 lbs etc. per regeneration BUT... they get you to forget that it only regenerates on average once every 7-8 days. Which one uses the least salt per week month or year? Also, if they are using only 11 gals/regeneration, and the other softener uses 35-40 gals/regeneration, which one uses the most water on the same week month or annual basis?
Hard water, actually the scale it causes, can not dissolve anything and actually coats metals protecting them from corrosive water.