Oil is oil, right? Wrong! Have you experienced the dread of searching for the correct engine oil in a foreign country where you don’t speak the language? Your little pocket dictionary won’t have the translation for, “I need API CJ-4 15W-40 diesel oil.” Pointing at the bottle on the shelf might work, but be sure you are pointing at the correct container!
To complicate matters, outside of North America you will find different prominent codes on the container related to other standards organizations like the ACEA (Association des Constructeurs Européens d’Automobiles), and you may see manufacturer-specific codes as well. Luckily, major oil suppliers tend to include a wide variety of codes on their labels, but you may find it to be information overkill.
A typical oil label on the back of popular Shell Rimula diesel oil (found widely outside of the U.S.) might look something like this:
“SAE Viscosity Grade: 15W-40.
API CI-4, CG-4, CF-4, CF; ACEA E7, E5; Global DHD-1, Cummins CES 20071, 72, 76, 77, 78; Cat ECF-1-A; DDC 93K215; Mack EO-M, EO-M+; MAN 3275; MB Approval 228.3; Renault Trucks RLD-2; Volvo VDS-3.”
What does it mean, and how do you find the correct oil for your engine, or does it matter at all? Addressing the last question first: Yes, it matters. But, depending on the type of engine, its age and what type of oil it was originally spec’d for, you can probably get away with a variety of oils that might not be ideal but likely won’t do any major damage. Notice all the caveats in that last sentence. No, it is not a good idea to stray far from the manufacturer’s guidelines — but yes, you may have to when tied up to the only fish boat dock with supplies of diesel oil.
The place to start is in the engine handbook that came with your new motor, which hopefully you have a copy of. If your vessel’s engine dates from the last decade or so, the recommendations might align pretty well with the labels on oil bottles. However, if you have an older boat, don’t be surprised if the official oil information doesn’t really match up with what you can purchase today.
For example, my Perkins diesel dates from 1978 and the engine handbook lists such things as Shell Rotella TX in 20W/20 or 20W/40, both of which are obsolete and no longer available. Plus, it only lists some MIL specs and nothing about API or ACEA stuff. You are unlikely to find MIL specs listed on a bottle of oil today.
Should you go hunting for antique engine oil? Obviously that is impractical, and in fact would be detrimental to your engine. Like many technologies, engine oil quality and performance have advanced significantly in the last 40 years. If you go direct to Perkins, or to many other major diesel engine companies, you will find them recommending their own brand of 15W-40 oil with a CI-4 or CJ-4 designation for a wide variety of engine sizes and types. The particular brand is less important than the API code and weight numbers.
A photo of the oil page from John Kettlewell’s engine manual that he keeps on his phone for reference when buying oil.
The multitool of oils
Why has this single oil (15W-40) replaced that page of alternatives in our old engine manuals? We first have to look at what the numbers mean. The first part, “15W,” stands for the oil’s viscosity (how thick and resistant to flow it is) in cold temperatures; the “40” is the viscosity at normal engine operating temperature, which is quite hot. The idea is that you need a thinner oil to flow easier when it is cold outside or when starting the engine, but the oil is engineered to increase in viscosity as it heats up in order to maintain the proper protection of engine components.
One thing to keep in mind is that the lower the W number, the easier your engine will start due to less resistance turning over with a thinner oil, and the oil will flow faster through the many oil cavities in your engine. A large portion of total engine wear occurs during startup and until the engine warms to proper temperature. During this period of time, the engine is somewhat “starved” of lubrication. Almost no matter how thin the oil is or how low the W number, the oil is simply too thick for good lubrication during startup. That is one reason why it is not a good idea to blast off under full throttle right after firing up the engine. Let it warm up a bit, and your engine oil and engine will be much happier.
Going back to our old manuals recommending things like straight 20W oils and sometimes a 20W-40, there was a good reason for that. Engineers knew about the extra wear caused by cold startups, so they spec’d oils that were light enough to allow for reasonable starts yet heavy enough for hot running. Oil technology of the day couldn’t produce multigrade oil that could last a full oil change interval and maintain the proper viscosities at both startup and normal operating temperature. Today’s 15W-40 diesel oils can do that, as has been amply proven in millions of over-the-road trucks, all sorts of commercial vehicles and other equipment that utilizes diesel engines that are very similar to marine ones.
To further complicate matters, there are many so-called “synthetic” oils on the market that can be easily recognized by their higher prices and dramatic advertising copy. I won’t get deeply into the pro and con arguments for synthetic oil, but suffice it to say it is probably superior to so-called “conventional” oil, though it is not required for most marine engines yet.
If you have an older engine, the handbook will not mention synthetics, but there will definitely be a possible synthetic substitute for whatever conventional oil you are running. Synthetics have better cold-flow properties — even at the same W rating — and they have wider viscosity ratings. For example, Shell sells Rotella T6 synthetic in weights of 0W-40, 5W-40 and 15W-40. Most boat owners would see little gain in going to anything below a 15W oil unless you start your engine in below-freezing temperatures.
You will get a lot of waterfront advice not to switch from conventional oil if you have an older engine with a lot of hours on it. You will hear horror stories about leaky seals and loss of oil. Synthetics do sometimes cause these problems on older engines due to improved cold-flow properties (when the engine is at rest) and because synthetics tend to be good at cleaning up old sludge in your engine, which may be acting as an oil seal.
If you are interested in switching to synthetic oil in an older engine, my recommendation is to try it gradually. Maybe start out by trying a semisynthetic oil, or by mixing your own by putting a mix of conventional and synthetic. See how it goes. Yes, it is generally safe to mix oils of different weights and types as long as they have similar API ratings like CH or CJ. In my experience, synthetics can provide a noticeable improvement in cold starts, even when temperatures are well above freezing, and synthetic oils have greater durability and create a potential for longer drain intervals.
The bottom line is that you will find it very hard, if not impossible, to quantify any gains by switching from the recommended type, weight and service designation of oil recommended by your engine’s manufacturer, which is very likely to be the aforementioned conventional CI or CJ 15W-40.
These API symbols found on lubricating oil packages provide information about viscosity and the types of service the oil was designed to provide.
Moving forward into the 21st century, you’ll find more and different acronyms followed by numbers on the backs of oil bottles. As stated previously, most of the newer oils are “backwards compatible,” meaning when your engine originally called for API CG-4 but all you can find is CJ-4, there is no need to worry. API categories for diesel engines, until recently, all began with the letter “C,” followed by the various letters of the alphabet. So, CF-4 came before CG-4, and CG-4 came before CJ-4, and so on. The gradual march down the alphabet related to improvements in oil specifications over the years, so you can be confident you are doing your engine good by purchasing the latest, greatest alphabet combination — until the last few years!
In the quest for lower engine emissions and greater fuel economy, modern diesels are becoming higher revving and hotter running, and they are beginning to utilize additional equipment that reduces soot, particles and greenhouse gas emissions. To keep these engines running properly, we started to see CK-4 and FA-4 oils in December 2016. Luckily, CK oils retain the backwards compatibility described above, but FA-4 oils do not! As a tiny warning about this, the diagram known as the “API donut,” which is found on many oil bottles, highlights the FA-4 designation in a red segment. The days when you can just grab any bottle of diesel oil at Walmart or a truck stop are over!
Another thing you will notice at your local truck stop or Walmart is that the shelves are heavily stocked with lower-viscosity oils like 5W-30 and 10W-30, which are engineered for easy starting of very tight-tolerance engines at lower temperatures, as well as for maximum fuel economy. Unless your marine engine handbook or manufacturer specifies those weights, be wary about trying lower-viscosity oils.
Few, if any marine engines, require FA-4 oils yet, but it is good to be aware of their existence since you will find them widely available wherever truck oil is sold, and those stores are great places to purchase quality oil for your marine engine. But watch out! Major manufacturers like Mobil and Shell are producing CK-4 and FA-4 oils that may be on the shelves right next to each other in very similar looking bottles. Typical clues that you are grabbing an FA bottle are phrases like “Advanced Fuel Economy.”
FA oils are all about pollution and fuel economy. These attributes are also needed in the marine engine world, and there is no doubt many boaters will soon be looking specifically for FA oil since it is spec’d in their engine handbook — or maybe it will be in the engine app!
One little trick I have found invaluable is to store a note on my phone with various filter numbers and engine oil specs so I have that information literally at my fingertips when I am in the oil store. I attach a photo of the oil page from my engine handbook to the note.
Good oil hunting!
John Kettlewell is the executive director of Sail Martha’s Vineyard. He has been cruising from Labrador to the Caribbean for more than 40 years, and he’s the author of The Intracoastal Waterway Chartbook.