My understanding is that gold toner does inevitably alter look and in the U.K. 1L which might do 50 8x10 prints cost about $80!!!

The real longevity toner is the brown sulphide one. Much cheaper than gold and probably better than selenium but it will change the colour of your print.

Sistan doesn't change colour but by how much does it increase longevity?

Key to all this in terms of cost to benefit is how long do you need the prints to last? So far it seems that properly processed and washed prints, even RC, have now lasted about 30 years and FB a lot longer

pentaxuser