Thoughts on oil change intervals (time, not mileage)

I have been changing the oil once a year, with less than 5000 miles. I have just switched to oil changing in the fall instead of the spring because my old mechanic friend figures old oil gets acidic and may damage important surfaces in the engine. I, too, drive it 30 km every month in winter. It seems there is at least a day in every month with an above freezing temperature.