depends on the circumstances, but the basic principles of your thinking are correct in my experience (2 years as a front line oil rep in the late 1980’s selling fuel/lubes to farmers/hauliers etc, 12 years in the industry overall). It depends where/how you bought that old fuel, originally? I’m certain forecourt fuel will have the appropriate additives all year round, certainly in 2013. The only occasion where fuel would possibly not have the appropriate winter additives would be commercial bulk purchases by farmers and hauliers into their own bulk storage tanks. More likely Farmers who would buy Gas Oil (tractor diesel), which is diesel without Duty added. Even in the old days I’m pretty sure I only sold winter additives for Gas Oil (tractor Diesel) not regular Diesel, but it was a long time ago.
In short, if you filled up at a forecourt pump, then almost certain no winter issues with the fuel.