What is one of the markings required on oil tank fillers for reciprocating engines?

The marking required on oil tank fillers for reciprocating engines is the word "oil." This specific labeling is crucial for safety and clarity, as it helps ensure that personnel can easily identify the tank's purpose. Proper identification minimizes the risk of using the wrong substance, which could lead to engine malfunction or damage. The term "oil" clearly communicates that the tank is designated for the oil needed to lubricate the engine’s parts, distinguishing it from fuel tanks that would contain a different substance, such as aviation fuel.

Other choices, while they may be relevant in other contexts, do not serve the same critical function of immediate identification of the tank’s content in the operational environment of reciprocating engines.