I have a question. I have a extremely low mileage W124 (300E 2.8) with only 56K miles so far, and currently only use it as a spare car and drive maybe 1,000 miles per year. Should I still change oil every 3 months?

I asked the mechanic who helped me change the oil today (10W-30 Castrol GTX), he said that there are some additives in the engine oil that would go bad after a while. Is this true? What kind of "additive" he is referring to and how come this additive would deteriorate over time and need to be replaced even though they were not used frequently? Thoughts or comments? Thanks!