What's the difference between Marine Engine Oil and Automotive Oil?

Marine Engines

Although marine engines are generally derived from automotive engines, they are not subject to the same working conditions. In normal conditions a boat's engine is put through much more stress than a car engine. Quicksilver and specially formulated marine grade oils for stern drives outperform automotive oils in almost all marine applications in two key areas: corrosion protection, load and heat resistance.

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Corrosion Protection

Typical automotive oils do not contain an adequate level of corrosion inhibitors needed by marine engines. An automotive engine will normally be run every day, whereas a boat engine (especially one used for leisure) will on average only be run once a week or less.

As an engine sits without being run, the oil gradually over time moves from the top parts of the engine into the sump. When an engine is first warmed up its vital that lubrication reaches all the bearings quickly. These bearings must remain protected with additional corrosion inhibitor, such as will be found in a marine environment.
It is true some of the best automotive oils contain high levels of corrosion inhibitor; however marine grade oil is specifically developed with the problem of long periods of immobilisation in mind.

Load and Heat

Automotive engine oils need to cope with an extremely wide range of outside temperatures. Marine oil does not need to contain additives to cope with such a wide range of use. It is important that Marine oil stays at a constant viscosity in order to give added protection to bearings when the engine is at maximum working temperature. This is more important than sacrificing protection to allow a freezing cold engine to turn over when temperatures would turn marine grade oil into thick less flowing oil.
To help boaters protect their investment and to ensure a positive boating experience, the NMMA and leaders in the marine industry have developed a four-stroke marine oil certification called FCW.

Products that have the FCW have met minimum testing and quality standards. There are roughly a dozen tests conducted to review viscosity, corrosion, filter plugging, foaming and aeration. In addition, the oil must successfully pass a 100-hour general performance engine test. Engine manufacturers such as Mercury Marine perform additional tests to ensure that our oils exceed these minimum standards. To learn about the NMMA go to www.NMMA.org.

All Quality Marine Grade four-stroke engine oils are FCW certified by the NMMA and are suitable for Marine applications. Quicksilver engine oils are blended from the highest quality oils available in the market. They are continually tested under the most rigorous conditions and must exceed Mercury Marine's minimum standards.

Air Temperature

When it comes to marine oils they are designed to cope with a narrower range of outside temperatures than automotive oils. To deal with this viscosity improvers (VIs) are added to automotive oils so they can continue to function at these temperatures.
With marine oils, it is advantageous for the oil to be thicker so VIs are not added.

Can I switch between full synthetic and synthetic-blend oils?

As long as the viscosity meets the service specs of the engine this is fine. All modern engines are compatible with both full-synthetic oil and synthetic oil blends. See your owner’s manual for your engine specs.

What is bore glazing?

Bore glazing it's a condition that usually occurs during the first critical hours of a marine engine's life. If an oil of too higher quality is used and the engine is not subjected to the correct loading (light loading is particularly bad), the honing marks become filled, making the bores smooth. With nothing to retain the oil in the bore, oil will begin to disappear through the exhaust. Symptoms tend to be high oil consumption, smoking and poor compression. It is possible to cure by adding a glaze-busting additive to the fuel. If this fails, re-honing may be required.

What is Sulphated Ash?

When oil burns in the combustion chamber, it creates ash that is very abrasive. This can be particularly bad where high ash products are used in engines used in high temperature, stressed conditions. The ash effectively polishes away the honing marks and leaves nothing to retain the oil in the bore. Once again, as with bore glazing, the symptoms are high oil consumption, smoke and reduced compression

What is Detergent Dispersing?

Detergency refers to the oils ability to keep marine engine components clean, particularly those in the hotter parts of the engine (pistons, rings, valves, etc.). The additive is referred to as a detergent. Dispersing refers to the oils ability to keep solid contaminants (i.e. soot, combustion debris, etc.) in suspension. This is necessary for two main reasons: firstly, it delivers the contaminants to the filter where the bigger particles are removed and secondly, it ensures that all the smaller particles flow out when the oil is drained, leaving the engine clean.

Why can't I use high detergent/dispersant oils in very old boat engines?

Usually this will depend upon the type of filtration used. If a simple mesh strainer is used on the pump inlet, low detergency/dispersion oils are a must. As previously described, dispersants keep all the 'rubbish' in suspension and allow it to circulate. Simple mesh strainers aren't efficient enough to remove it and so it continues to circulate causing damage.

What's the difference between a multigrade and a monograde?

The main difference between these two types of engine oil is their fluidity at cold start. Multigrades, such as 10W/40, 15W/40, 5W/30, etc., flow more easily and are therefore pumped round to the critical components much more quickly. Historically, the 'W' stands for winter. As well as this important feature, Multigrade also must provide a protective oil film at higher temperatures when the engine has warmed up. Monograde on the other hand provide a very good oil film at working temperatures, but their cold start properties are poor. To overcome this it used to be the practice to put a thin monograde, such as a SAE 30, in the engine during the winter and a heavier monograde, such as a SAE 50.