212. While underway, a slow-speed diesel engine lube oil sump level slowly begins to decrease. Which of the following should be checked?

A slowly decreasing lube oil sump level typically indicates a leak from the lube oil cooler into the cooling system. This internal leak allows oil to be carried away with the cooling water discharge, resulting in the observed sump level decrease. Other potential causes, such as piston oil scraper ring wear or standby pump operation, do not directly explain this symptom.