What must oil tank fillers on turbine engines be marked with, according to regulations?

The marking of oil tank fillers on turbine engines with the word "Oil" is crucial for safety and operational clarity. This designation ensures that personnel can easily identify the correct type of fluid to be added to the system, thereby reducing the risk of contamination and potential damage caused by using incorrect fluids.

In aviation, where precision and safety are paramount, it is essential to clearly label components to facilitate maintenance and refueling processes. The specific marking helps prevent mix-ups that could lead to severe mechanical failures or hazardous situations. By designating oil tanks specifically, maintenance crews and ground staff can stay organized and prevent costly mistakes, ensuring the turbine engine functions reliably and safely.

The other options, while relevant in their contexts—for example, fuel is important for operation, capacities are significant for understanding the amount of fluid required, and pressure levels are critical for performance—do not directly address the necessity of identifying the oil filler. Therefore, the regulation's focus on marking the filler with "Oil" highlights its priority in maintaining operational integrity.