The PEG 600 would be pretty viscous too - it will really depend on what you're using it for. It's a rule of formulation that the desired aspect of the material is what determines the substitute - if you wanted to make a suspension, but make it sweet, you'd have to replace the peg with Glycerine, for example. if you're just using it for the suspension effect - well, 300,400 would be about the same...600 is another story...