Mine are 5ml oral syringes and they still get sticky after a while.  I think its how you wash them and treat them.  Like I said when they go in the dishwasher, I think the heat and the detergent does something to them.  Your best off pumping them out with a bit of hot weak dishwash solution in a mug and then leaving them apart to dry out.  If you leave the plunger in the syringe it will stay wetter for longer and just rot the rubber I reckon.  That’s what I used to do and now they do last longer if they are washed by hand.