Hi Greg
Given the age of the fuel, it is likely that fuel has lost some of it's volatile components and these will not be replaced simply adding any fuel additive.
Although draining the old fuel and replacing it, is an option, you can also consider simply adding fresh fuel. A good ratio would probably be 50%, so say 10L in this instance. Then run the vehicle for a while and if the intention is run the vehicle only intermittently, use our ULTIMATE FUEL STABILISER at the recommend treat rate.
Note however that fuel quality will still reduce over time and so a plan to use up all the fuel in say a 3-6 month period should be enacted.
Also note that fuel may not be the only reason why the car is running roughly eg poor tuning, injector blockage, etc, can also impact this.
Cheers
Costa
Penrite