Bluheaven, I would be careful. I had a 2.5 year old bottle of Z3 which I used because everyone said it had a shelf life of 3 years. It didn't go on or come off as easy as it did when it was new and also left streaks that were hard to remove. I had to wash it with Dawn again and buy a new bottle of Z3.
Open up the bottle and check to see if the liquid is smooth. If it looks cakey or lumpy, I personally would toss it. The hole on top of the bottle may look a little cakey, just remove that with a knife. You want to get a good look at the majority of the liquid which is at the bottom. I hope this wasn't too confusing. Let me know.
Also, Zaino has made a newer formula which looks a lot thinner. I just bought some product a couple of weeks ago and the liquid in Z3, Z5 is definitely more thinner than the ones I had from a couple of years ago.
Erik