What oil should I use?

The two most common questions that I still get are: 1. What oil should I use in my aircraft engine? And 2. Are different brands and grades of oils compatible?

In this column I will limit the discussion to certified engines, such as Continental, Lycoming, radials and older certified aircraft engines. For these engines there are four different choices.

First is straight mineral oil that meets the SAE J-1966 and Mil-L-6082E specification. The second is single grade ashless dispersant or AD oils that meet the SAE J-1899 and MIL-L-22851 D specification. The third type are multi-grade oils that meet the same SAE J-1899 and Mil-L-22851D specification. The last type of oils meet the same spec as the AD and multi-grade oils, but also contain the Lycoming LW 16702 additive or equivalent and are referred to as anti-wear oils.

So are these oils all compatible with the other choices? Well, usually yes, but there are some notable exceptions.

The straight mineral oils are usually used only for break-in of new engines. To determine which oil to use when breaking in your new engine, always check with your manufacturer or rebuilder. (I also discussed this in one of my previous columns, which ran in the Nov. 23, 2012, issue. It’s also available at GeneralAviationNews.com.)

Some people will use straight mineral oil for the entire life of the engine, which is legal. But this can lead to problems if the aircraft is sold and the new owners change to an AD oil. The AD oil can loosen up some carbon deposits, which can plug screens and filters, causing oil flow problems and even engine failure. Problems may only occur in, say, 20% of the change overs, but that is a significant level.

The big question is what is the limit in hours after which it is not advisable to switch to an AD oil? There is no hard and fast limit here because it depends on how the plane was flown and maintained. My best guess would be to not change over an aircraft after 200 to 300 hours — and even then monitor the oil screens and filters very carefully.

If you lose a cylinder mid TBO, you can switch from AD oil back to a mineral oil for a few hours, and then switch back to AD oil. The problem occurs from extended operation on the mineral oil, so always determine which type of oil was used when buying a used aircraft.

Another note is that many rebuilders do not recommend using the anti-wear type oil during break-in, except for engines like the Lycoming O-320-H, which must use the additive or one of these oils.

Other than these exceptions, all approved oils are compatible. You can switch brands and grades without fear of harming your engine. You may see some change in oil consumption when switching from a single grade to a multi-grade. Multi-grade oils tend to leak better, but on a good tight engine they usually reduce oil consumption. I have heard from several pilots who bought a case of mineral oil for break-in and now have no use for it. One option is to add a quart to each AD oil change.

As to which oil to use, that is a long-standing debate. Multi-grade oils have improved flow characteristics in cold weather, and you can use them year around. But a significant number of engine rebuilders have a slight preference for single grade oils in the warmer months.

If you live in a colder climate or fly into a colder climate, I usually recommend using a single grade in the summer and a multi-grade in the winter. If you live in a warmer climate and do not start your engine below about 40°F, you can use single grade oils year around.

If you operate a radial engine, the smaller ones with displacement less than 1500 cubic inches, they seem to work well with multi-grade oils. However, the large ones, especially if you do any reverse loading by the prop, seem to be better served by single grade oils.

Finally, should you use an anti-wear oil? This depends on how you fly and where you live. If you live in a more humid climate and do not fly regularly, then I would recommend using an anti-wear oil. Since radials have roller lifters, an anti-wear oil will not show a significant improvement in these applications.

Comments

  1. I don’t voice my opinion much but who is Craig? I can tell you personally that Ben didn’t spend his time at Shell pumping gas. I worked with him for 8 years at Shell Research. WE were working with oils, even synthetics back in the early 1970s. Most of the basics havn’t changed all that much. I’ll stand with Bens opinnions and knowing what he did and how he did research. By the way Craig on a nother note, go to the website PureGas.org and you’ll find a lot of stations that sell unleaded gas without ethanol, especially anywhere close to the water and outboard engines. Maybe a little Research might help next time.

  2. Aren’t you glad that people such as Craig exist? Takes that egotistical 5 percent to make you
    appreciate the 95 percent who carefully think through their comments before going off half cocked, and attacking someone whose knowledge obviously greatly exceeds theirs.

  3. Ben,
    I value your experience and advice, whether it happens to be backed up with a spreadsheet or not. Wondering if you could add any insight to the argument on whether mixing weight oils causes problems or not? If the engine has a 10w-30 in it and you could only find a quart of 10w-40 to top off with, is that okay to do on rare occasion, or going to get one into trouble?

  4. TomSpann says:

    We spent a week at Cessna Wichita in August 1968 attending their Red Carpet school and following our new 421 in its final days of checkout and delivery. During that week we not only learned the aircraft systems but one of the weeks topics was which oil to use. The Shell AD 50 was the oil of choice by Cessna at that time. I don’t remember if multi-viscosity was available n 1968. There was no discussion comparing brands. We kept Shell AD 50 on board in case we needed it. All HBOs did not carry Shell products. Is mixing brands OK?

  5. Joe Corrao says:

    Craig, when I was in college, more so in grad school, and for a few years thereafter, I too demanded that every number be supported by a spreadsheet. Its what Typo As do, I guess. As I’ve reached maturity, I’ve learned that men and women of character who devote themselves to their discipline often acquire a knowledge of it that goes beyond spreadsheets. Such peoples’ “estimates” are often better than spreadsheet results because spreadsheet results are based only on the available information, and information is always imperfect; the true espert’s “estimate” is based on information accumulated over time and distilled. We call that distillate, “wisdom.” It is something young folks usually can’t have because the distillation process takes time. One day, if you come to regret your momentary impertinence, please take heart; wise people forgive easily. They were all young once, and estimates indicate that most were impertinent at the time.

    • Yes Joe, I’m over 50 with grad school too, yadda yadda, and the whole article seems like an editorial with NO facts thrown in. This was all an opinion with a made up percentage to boot. Not impressed.

  6. Expert? Who states things like this? “Problems may only occur in, say, 20% of the change overs, but that is a significant level.” What back up is there that it is SAY 20% and not SAY 25%. Please show ANY proof for this number and it was not just pulled out of … the air. 33 years with Shell Oil does not make an expert, he could have been pumping gas.

    • SkyMachines says:

      Craig, just because you aren’t familiar with Ben’s work (evidently), doesn’t mean you need to question everything he writes as only “opinion.” First off, it’s a column, and it’s exactly opinion that he is supposed to put forth. Secondly, if you were a regular reader of GAN, you would know he’s written this column for years and, when he worked for Shell, he was not “pumping gas” but was involved directly in aviation lubricants. I therefore value his advice.

      That said, Aviation Consumer magazine has done several relatively-scientific articles on engine oils over the years. Their conclusion is that Philips XC 5w-50 with an additive such as Camguard offers the best all-around protection. That is good enough for me.

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