Question
potassium sulphate, can this be used to fertilize avocado trees or is there something better

Answer
If you have an Avocado Farm, using Potassium Sulphate (K2SO4) would be an important choice you could make to address excessive chloride readings in your soil.

In one test, applying Potassium Sulphate decreased Calcium levels in the fruit and appeared to lower the quality of the harvest.  Lowered Potassium and higher rates of Magnesium in individual fruits have been linked to improvements in disease incidence.  Apparently it is the Calcium/Potassium ratio which drives the incidence of disease and fruit quality.  There is a great deal of study, in fact, on the interactions of these different minerals and the effect on fruit.

Bear in mind, different Avocado rootstocks absorb nutrients at different rates.

Do you have an Orchard, or a single Tree?

There are several studies on the internet documenting various studies on this.  One is the Avocado Source website (http://www.avocadosource.com/Journals/AUSNZ/AUSNZ_2005/HofmanPeter2005.pdf), a library of Avocado research.  Search for Potassium Sulphate and you'll find many articles addressing your question.