I am in agreement that it is only certain flavors that cause the problem. I have found that I can help extend the life of a syringe by rinsing it with pure grain alcohol/ethyl alcohol/ever-clear between each and every flavor and before storing. I have read of others who have a dedicated syringe for those "problem" flavors, and just know that it will need frequent replacement, but will not have the difficult/sticky operation for all the other ingredients. Or, if your budget allows, use a glass syringe like these.
Or you could be crafty and convert your weights into drop equivalents and stop using syringes... just sayin'