I allways use GG4 or glimmer clean if they are out of gg4, the gg4 works out cheaper.
if you live in a soft water area you should use GG3, if hard then you are right with GG4 but up the dilution forget the 2 Ml per ltr or even the 4 ml per ltr up to about 6 Ml per ltr and forget the sorbo.
I have found diferent rubbers have an efect, im using ettore at moment no probs had probs with unger rubber though