When toiet flushed water rises then slowly drains to correct level leaving deposits.if bath emtied or shower used toilet water gurgles.both done at same time results in both draing very slowly.when downstairs sink emptied toilet again gurgles(toilet upstairs)tried drain clearing devices and products,no difference.

You have a downstream blockage or possible blocked vent. Putting drain chemicals in doesn't usually work for blockages that are not right at the sink or tub. Have the drain and vent system checked by a plumber.