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The MerCruiser line of motors is the power plant for many models in the Mercury Marine line of boats. The MerCruiser 3.0 engine, which features four cylinders and is rated at 135 horsepower, is named because of its displacement of 3.0 liters. The manufacturer recommends that its own MerCruiser SAE 20W-40 Full Synthetic Engine Oil be used.
The recommended SAE 20W-40 is designed to function at high temperatures and under high-motor-load conditions. The 20W-40 is the engine's viscosity rating, with the first of the two numbers and the corresponding "W" indicating the winter rating of the oil. Thicker oil (those with higher numbers) is designed to maintain pressure at high temperatures and loads like those encountered during summer months spent racing around the lake.
Synthetic oils have many of the same lubricating components as conventional oils, but instead of being refined from fossil fuels are made during chemical reactions. The synthetic oil has the capability to perform well in high temperatures, with some manufacturers claiming that they can withstand an operating temperature of 400 degrees Fahrenheit. "Because a synthetic oil's molecules are much more consistent in size and shape, they are better able to withstand extreme engine temperatures," writes Car Craft magazine.
Mercury Marine recommends several other oils with similar properties if its own full synthetic is unavailable. Acceptable alternatives include Mercury/Quicksilver 25W-40 synthetic Blend, Mercury/Quicksilver 25W-40 conventional or other brands of 4-cycle oils. According to Mercury, a high-grade, straight-weight automotive oil with detergent can also be used. Non-detergent oils are not recommended in MerCruiser 3.0 motors.