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Choosing Oil for Marine Engine

Tips to consider when choosing an oil for your marine engine

Prolong the life of your engine
Jim Merten Jr., President of Merten Marine LTD

“Synthetic oils are more expensive, but because they blend with ethanol and offer friction reduction, synthetic oils may offer better protection for your engine overall. Conversely, straight petroleum oils are more affordable, but may result in reduced durability.”

Whether you've owned your boat for a while or are a relatively new boat owner, sooner or later your vessel’s engine will need an oil change. You can certainly bring it in and have your local dealer handle the oil change for you, but if the "do-it-yourself" bug has bitten you, you’ll want to make sure you’re using the correct oil in order to get the best performance from your engine.

The best advice is to always refer to your owner’s manual. If you purchased the boat new an owner's manual automatically comes with it. But, if you are like many people who buy a used boat, you may not have that owner's manual. Like car engines, all oils are not the same. Beginning with the basics you need to understand there is a huge difference between car and marine oils.

The National Marine Manufacturers Association (or NMMA) tests engine oils for marine use – either 100 percent synthetic or partially synthetic or straight petroleum oil – to gauge their lubricating properties and, based on their tests, gives certified recommendations designed to prolong the life of marine engines.

Synthetic oils are more expensive, but because they blend with ethanol and offer friction reduction, synthetic oils may offer better protection for your engine overall. Conversely, straight petroleum oils are more affordable but may result in reduced durability.

It’s important to note that there are different oils that go with different types of engines. NMMA-certified oils include: TC-W3, FC-W and FC-W (CAT). TC-W3 is used with two-stroke engines, FC-W with four-stroke engines, and FC-W (CAT) with four-stroke engines that have exhaust treatment catalyst systems.

The four-stroke and the stern drives have oils that are similar to what you buy for your cars, but they’re actually blended backwards from a car oil, according to what Mercury Marine says.

Say your car uses 10W30 oil. The first number on a car oil is the base weight of the oil. If it’s a 10W30 they have emulsifiers to make the oil think it’s a 30 weight after the engine warms up. The car has air going underneath the engine and cooling it off.
In the marine application, your engine is housed inside the boat and there is no air flow. Because there is no air flow the engine relies only on the water coolant flow that is picked up out of the lower unit. The bilge temperatures are really high compared to a car. In comparison to car oil, marine oil is designed to blend it backwards. So if you've got a 10W30 the base oil is 30 weight for the higher heat, and they have emulsifiers to trick the oil to make it thinks it’s thin when you first start it up. When the engine is cold the oil will reach all the proper internal places for lubrication.

Certainly, if you find yourself in a situation where oil is critically needed, any oil is better than no oil. Once you are in a position to change the oil, put in the correct kind of oil manufacturer recommended for your engine and this should be done immediately.

Many times the outboard oils sold at gas stations just meet the TC-W3 ratings. The outboard oil produced from marine engine manufacturers are blended beyond the TC-W3 rating. They are the companies that warranty the product they produce.

I have seen cases where Mercury denied claims even though the engine was still under warranty because "do-it-yourself" people used sub-standard oils. When a claim is submitted under a warranty, it is not uncommon for the manufacturer to request the parts; they have a way of analyzing what was put in the engine. So it is important to spend the little extra money for a higher grade oil that is NMMA certified and recommended by your engine’s manufacturer. This actually becomes more critical as an engine gets older. Parts become worn and have the tendency for a leak. This is where the better quality oils keep your equipment stronger for much longer.

Two Additional Points:

  1. When changing your oil, it is a good idea to change the oil filter at the same time to keep your engine running at its best performance.
  2. Avoid adding different types of oils at the same time in your engine (only exception is in urgent/emergency cases). If switching to a different type of oil, this should be done when doing a complete oil change and not for regular top-offs.