The correct answer is C) Shift over to the standby heater and monitor contaminated drain tank for additional traces of oil. This is the correct first step because finding oil in the main fuel oil heater steam drain system indicates a potential issue with the fuel oil heating system. Shifting over to the standby heater allows you to isolate the problem and monitor the contaminated drain tank for any additional signs of oil. This helps identify the source of the issue before taking more drastic actions like changing the fuel supply or blowing down the boiler, which could exacerbate the problem if the underlying cause is not addressed first. The other options, while potentially appropriate in certain circumstances, are not the optimal first step in this scenario, as they do not help diagnose the root cause of the oil contamination.