Re: using automotive oil for a 4.3 stern drive
Be careful with advice to use automotive and diesel oils in your Mercruiser. Marine engines operate at very high speeds and loads when compared to passenger and diesel engines. Mercury's 25W-40 is designed specifially for marine use.<br /><br />Off-the-shelf automotive lubricants contain polymers which give them their multiviscosity properties. These polymers can make a 10 weight base oil have the added viscometric properties of a 30 or 40 weight. The viscosity of these common oils tends to change with the flow rate of the oil due to the "shearing" or breaking apart of the long "strands" of the polymer. These are called non-newtonian oils. Under high speed and load of a marine engine, some of these polymers experience permanent shear loss resulting in a subsequent change in viscosity. This can cause low oil pressure and engine damage. Probably what member xltier experienced.<br /><br />Mercury's 25W-40 oil is a Newtonian fluid. This means the viscosity does not change with the rate of flow. It is formulated without polymers. The multiviscosity properties are obtained by blending several very special base stocks that eliminate the need for the polymers, which are commonly used to give automotive lubricants their multiviscosity properties.<br /><br />Marine engines also experience unusual extended periods of idle or trolling. This creates a need for the oil to continue to function even when diluted with fuel and moisture. Mercury's 25W-40 has a unique additive package designed for marine use so it can deal with potential problems caused by these operating conditions. Some of the polymers used in automotive oils are hygroscopic, or water gathering. They can form a light brown emulsion usually observed in the crankcase breather areas. The wet marine environment tends to exacerbate this problem. This further inhibits the automotive oil's imulsified polymer to function as a multiviscosity fluid.<br /><br />Mercury recommends the special 25W-40 oil for all temperatures. If it is not available they recommend a straight 20W at temperatures below 32'F, a straight 30W at temperatures between 32'F and 50'F, and a straight 40W at temperatures above 50'F. In the unlikeley event none of these oils are available, their last recommendation is to use a quality 20W-40 or 20W-50 API oil.<br /><br />According to Mercury:<br /><br />IMPORTANT: The use of non-detergent oils, multi-viscosity oils (other than Quicksilver 25W-40 or a good quality 20W-40 or 20W-50), synthetic oils, low quality oils or oils that contain solid additives are specifically NOT recommended..<br /><br />The best bet is to follow the manufacturer's recommendations.
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