John Davidson, 47, has refused to buy any products sold by A.G Barr since production ceased of the old recipe, on January 4, 2018.
The self-employed electrician was devastated when the sugar content of his favourite drink was controversially axed from 10.3g to 4.7g per 100 millilitres, making A.G Barr exempt from the sugar tax.
But John, from Cumbernauld, North Lanarkshire, was elated when he discovered an unopened can of the original recipe Irn Bru in a disused pub where he was working.
He found the can, which expired in March 2017, in a disused social pub in Stranraer, Dumfries and Galloway, and asked the landlord if he could keep it.
This week he finally opened it, and said it tasted just the same as if it had just come off the production line from the factory, which is minutes from his home.
Dad-of-two John said: "I was carrying out some work for one of my customers, it's a social club which had closed down for some time.
"I was meeting the owner, and I saw it when I went in, I'm always on the lookout for old cans of Irn Bru.
"I spotted it right away because of the old label.
"I looked at the expiry date on the bottom of the can and saw how old it was, and asked if I could keep it and he said 'no problem'.
"He looked at me strangely, like 'why would you want to be drinking an old can of Irn Bru?'.
"But I've got little glass bottles at home that were best before December 2018 and I'm still drinking them, and they're fine.
"Sugar is a natural preservative."
The sugar content was controversially axed from 10.3g to 4.7g per 100 millilitres, making A.G Barr exempt from the sugar tax.
However, since the new recipe Irn Bru was rolled out in January 2018 John has boycotted any product made by A.G Barr.
John added: "It was just as good as if it had just come off the production line.
"I'm sure lots of people will be very jealous, a lot of people haven't had the old recipe for years now."