Marine Oil, Filters, and Water Separators vs Automotive

As a part of general boat maintenance for inboard or outboard boats, one essential maintenance task to learn about is engine oil. When it comes to boats, marine oil and filters are slightly different than those found in cars.

While the differences can be pretty subtle, it’s worth knowing the difference prior to this important step in winterizing and storing your boat I discuss in this post.

Without going too deep in the weeds, in this post, today I wanted to break down the basics for any beginner when it comes to understanding marine oil and filter products used on inboard or outboard boats.

Marine oil filters vs automotive oil filters

When it comes to the way oil filters are made, marine oil filters really only differ from automotive oil filters in the sense that marine oil filters are typically more corrosion-resistant. This can make a huge difference for boats that stay in and around saltwater but is less of a concern for boats in freshwater.

While an oil filter is important to keeping your engine clean, even more, important is regular oil changes, since it’s not uncommon at wide-open throttle or in colder temperature for oil to bypass the filter completely.

That being said, many boat owners use automotive filters, but depending on where you’re launching it, it’s probably worth going with a marine-grade filter.

A few top marine oil filter brands to consider

When it comes to filters, many of these are brand-specific and cost anywhere from $10 to $30 for both inboard and outboard-specific filters.

Many enthusiasts swear by using automotive filters for marine applications, but I personally would err on the side of caution and spend the extra bucks for corrosive protection. You also risk voiding the warranty on new boats if something were to happen.

There are a couple of horror stories associated with using Fram automotive oil filters on boats I’ve heard, however many others have never had issues.

It’s always a safe bet to use the OEM recommended oil filter and oils. Many are brand-specific for boats anyway (like Mercury) and can be found on Amazon. Here are a few common brands to check out. As always, you’ll want to make sure they are compatible.

If you don’t already have one, you’ll also want to pick up a marine oil change pump if you plan to change the oil on an inboard.

Differences in oil filters between inboard or outboard motors

Keep in mind that it’s important to find out what oil, oil filter, and fuel filter (and water separator) specifications your boat requires since inboard and outboard motors are obviously different. Some are longer/shorter than others.

Depending on the boat you own, you’re likely to find a video on YouTube on how to change your filters and fluids, or by looking through your boat’s manual.

The difference in marine oil vs. automotive oil

When it comes to the oil itself, the main difference in the oil itself will be that marine oil contains a higher percentage of additives that mainly help to combat rust and corrosion boats are more exposed to.

The additives in automotive oil are typically 10 to 20 percent, marine oils can contain around 20 to 30 percent.

Anti-wear additives also are normally added due to the nature of marine engines vs. car engines. Since boats are more exposed to throttling at higher RPMs (around 5,000 at 30 mph!) as opposed to cars where the RPMs stay in the 1-2,000 range most of the time.

Both the Quicksilver, Mercury, and AMS oils are popular marine brands, but others like Mobile-1 synthetics many people swear by. Again, this is a matter of your boat’s age, warranty, and what you feel comfortable using.

Look for oils certified by the National Marine Manufacturers Association

Since marine oils obviously have different weights and ratings, you should read up on the NMMA website here on why oils and additives are so important for marine engines. The video below does a good job at explaining why the right NMMA-certified oil is important for boats as opposed to cars:

What is a marine fuel water separator?

Without going into too much detail, a fuel water separator’s main job is to block water starting from the fuel tank of your boat, from entering the sensitive parts of your boat’s engine, causing corrosion and potentially damaging fuel.

Racor is the leading manufacturer of these types of fuel filters, which due to their construction, also fall into the category of fuel water separators. A fuel water separator helps to separate water that may have entered your tank prior to being sucked into the engine.

These are common with diesel engines in boats due to the properties of diesel fuel tending to create more pockets of air where water can enter. In any case, topping off your tank is another best practice to avoid this issue.

Outboards only: Lower unit gearcase oil/lubricant

Okay, so a couple more things to know about marine oil and greases. For outboards, or sterndrive boat engines, the lower unit oil should also be drained and changed from time to time. This is also referred to as gearcase oil or lube, and is changed from the bottom of the lower unit by pumping it into the unit after draining.

Below is a simple video to help illustrate this process. In this case, the lube is pumped into the lower unit, but not all engines require a pump for this application.


While you’re at it, it’s also a good idea to understand how to use a grease gun or tube to grease your grease fittings/zirks for those outboard motors to improve the performance of steering/tilt mechanisms.

Conclusion

Now that you know the basics of the different kinds of marine oils, oil filters (and oil separators), in addition to other types of fluids I covered, hopefully, you have a basic understanding of what’s required as a boat owner.

If you decide to winterize or store your boat as I discussed in this post, you can always ask your local marine questions about this process if you need expert help on any of these oil-related or filter-related tasks you are considering tackling in the future. Hope it helps!

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