• The manufacturer requires a specific lubricant for the MerCruiser engine.Jupiterimages/Photos.com/Getty Images
  
  The MerCruiser 4.3L is an inboard boat engine, manufactured by Mercury Marine, used to power the stern drive of the vessel in which it is mounted. Mercury Marine has specific recommendations regarding the type of oil to be used in its MerCruiser engines.
  
  

Recommended Oil

• Mercury Marine, the manufacturer of the 4.3L MerCruiser engine, specifies that the owner use Mercury MerCruiser Full-Synthetic Engine Oil, 20W-40, with a NMMC EC-W rating. This specific oil is recommended by the company for all MerCruiser engines, including the 4.3L. It is available at many marine service/parts centers.
  
  

What the Ratings Mean

• An oil with an EC-W rating is specified by Mercury Marine for use in the MerCruiser 4.3L. Ratings are given by the NMMA (National Marine Manufacturer's Association). To achieve this rating, the oil is laboratory-tested for viscosity, corrosion, filter plugging, foaming, and aeration. The oil must pass these tests to the NMMA standards, including a 100-hour general-performance engine test.
  
  Mercury Marine recommends a full-synthetic oil for use in all of its MerCruiser engines. Synthetic oil uses a created, or synthesized base, and is of much higher purity than petroleum-based lubricants. It is made with smaller molecules and fewer impurities than is regular oil Synthetic oil can stand up to heat better than conventional oils. It is also able to withstand colder temperatures.
  
  

Substitutes

• Mercury Marine suggests that some of its Quicksilver oils with a 25W-40 weight may be used for a short term if MerCruiser Full-Synthetic oil is unavailable.
  
  A variety of other companies also offer 20W-40 Full-Synthetic Marine Oil with the required NMMA EC-W certification.