[QUOTEIlford states: "Use a single fixing bath plus a washing aid. The number of prints through the single fixing bath can be increased to approximately 40 8x10" prints per litre working solution." in this document, page 3, right-hand column, point 3. This is in contrast to what they say is the capacity without a wash aid: "Fix only a few prints before replacing the fixing bath (approximately 10 8x10 inches prints)."][/QUOTE]
The ilford 40 print reference is for processing to a commercial archival standard (7-10) years. If you use a wash aid you may extend the Commercial capacity of a one tray fix. The fix has a capacity to process maybe 80 to 100 prints but as more prints run through the fix the more silver thiosulfate compounds build up reducing print life.
If you limit fix capacity to (10) 8x10s per L, you don't need a wash aid to obtain optimum permanence without toning. If you tone, you must use a wash aid for optimum permanence.
I'm a simple guy. If your capacity is high, you can explore ways to recover silver. There seems to be a desire for one fix tray and higher capacity. One could just use RC paper which allows higher capacity and uses less water.