Back when we rarely used the gun I never had any zim spray go bad but we did get a 55 gallon drum of zim spray fluid from a shop that went out of business and it sat unused for several years and when I pulled some fluid out of it to use one day it clogged up the gun. Every moving part got gummed up and I remember it smelling more like kerosene than typical zim fluid. I still can't believe that shop bought zim fluid by the barrel, I'm sure that was part of the reason why they went under.
Back when I was operating the press we would go 6 months without using a gallon of zim spray...I miss those days. Now we use a gallon every 4-5 weeks. I can't stand the sound of the gun going off and if I hear it for more than 5 seconds I go see what's going on.