Water softener salt and dish washers

Our water is mildly hard, not hard enough to make a whole-house water softener necessary. The only place where it's necessary is the dish washer.
Our dishwasher (Miele) has a built-in water softener which works great.
However, the salt the thing needs is getting a bit hard to obtain locally.
I've always wondered whether it was possible to use bulk water softener (WS) salt rather than the "special salt" the manual refers to. The "special salt" (a european brand "SOMA", not made by the DW manufacturer) comes in "single load" boxes (last about 2-3 months), and is quite expensive compared to bulk WS salt.
Right now obtaining SOMA salt is at least a 2 hour shopping trip.
Salt is salt, more or less. You'd think. Does anyone know if it's possible to use bulk WS salt in these units? I'd guess that the premium stuff (refined to have less dirt and crud) would be best, but I'd like to hear a more expert opinion.
Chris Lewis, Una confibula non set est
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