The Cessna 120-140 Association Forum

I thought this would be a fun discussion/brain exercise.

On a newly overhauled Continental that is typical in our birds, for break-in the continental overhaul manual emphatically states: service the engine with mineral oil (MIL-C-6529 Type II) of the appropriate grade for ambient temperature. The operating instructions (Section VII) also state that Continental engines are certificated on straight mineral oil. Detergent/ashless-dispersant oils meeting MHS-24B are approved for normal operation, but the initial servicing and break-in call for mineral oil.

Current Continental guidance (in SIL99-2 series) specifies:

Break-in oil: SAE J1966 non-dispersant mineral oil, or MIL-C-6529 Type II corrosion-preventive mineral oil (limited to ~25 hours or 6 months).
Normal operation: Ashless-dispersant oils meeting SAE J1899 / MHS-24B.

X/C falls into the Ashless Dispersant category and is an MHS-24B oil, so it is approved for ongoing use but is not the primary recommendation for the initial break-in period when rings need to seat.

BUT...
On most of the continental forums and in the 120/140 group, Phillips XC 20/50W is suggested as a great break-in oil.

What are your thoughts and opinions? I'm about to pull the trigger on a case of 20/50 XC (which I run normally anyway), but want to hear from the experts. My A&P doesn't care which I use and says both are fine. I'm curious what your experience is.

From years in the business it seems as if Mineral oil is the best way to go for break in oil. It is and has been an industry standard for longer than I've been involved in the business. (1981)

Here's what I've experienced first hand.
1. Aviation consumer was a publication that offered advice on aviation products(among other articles)
Approximately 20......(30??) years ago, they employed a writer who was a fierce advocate for not using Mineral oil for engine break in. I forget his exact arguments, the only phrase I can remember (almost word for word) was "Mineral oil is for worming horses". Approximately 1-2 years later (after the offending writer had left the scene/?) came the sorry reports from the many distressed owners who had had to pull cylinders, have them re-honed and re-ringed due to not being broken in......at all. The "letters to the editor" were direct and very negative. I don't think this one issue assisted in Aviation Consumer's publication end days.........but it probably helped.

2. The industry standard is to use mineral oil, even the much praised (and despised ?) Mike Busch clearly recommends mineral oil for break in. Seriously, go to Savvy Aviation's website, he has great articles on oil, one of which I still hand out to customers who want to know what viscocity and brand to use. Few people, companies, groups etc have as much piston data as Savvy does. In this particular case I agree with Mike and Savvy on both Break in oil and also his recommendations of AD oil A. utilizing straight grade Aeroshell with camguard as a first choice, if not then B. Phillips XC20W-50 oil with camguard if you need Multigrade (I use multigrade XC in my Michigan climate with the vast temperatures swings and where pre-heat is not always available)
Also Savvy makes an excellent argument for not using synthetic or semi synthetic oils in our aircooled engines.

3. I wasn't aware people still are advocating going staight to AD oil. I would be interested to know if it's single owners or shops that overhaul engines and have to warranty and support the warranty of their products. Remember on the internet......everyone is an expert and on FB.....considerably more so.

4. When I ran a decent sized MX shop including a very busy 141 flight school and a busy 135 charter with 5 C-414's (for the piston power, not turbine, that's a whole different discussion)............... IF..........we replaced just one cylinder ....or maybe even two cylinders on a mid time or higher engine we didn't switch back to Mineral we just put in whatever oil the customer was using. The local engine shops, Poplar Grove and G&N were on board with this decision. It seemed to work out just fine. Read into this what you will?
4A. We had excellent results with break in of mineral oil for the first 25 hours and AD after that. I don't recall ever having to pull a cylinder in the decades that I was in that business for high oil consumption or "glazed bores". Of the hundreds of piston engines that went through the shop for our own aircraft where we had full control, only 2 did not make it to TBO, one was a 414 that failed the prop governor drive shaft (that was a wild ride.........ever had a runaway prop on a descent!!/? ) the other was a flight school trainer with faulty Lycoming cylinders.
4B. Over the years I regularly attended mx seminars with other DOM's from all accross the USA and also other countries and I never heard anyone mention that they didn't use mineral oil for break in. This topic came up often enough ......
4C. I regularly query engine shop managers (who have to support and warranty their product) and also petroleum engineers for Aeroshell and Phillips and Exxon(when they made Elite) and Camguard etc. They all stated due to the properties of Mineral vs AD to use Mineral initally....... They had "Chemical and Metallurgical reasons".(which I can no longer remember....)

5. Why even think about a "questionable activity" when there is so much history for using the "industry standard?" (go back to number 1!!??) I do suppose that not everyone that took the advice listed above to start right out with AD oil had un-satisfactory results......like any poll or survey or questionaire etc, generally it's the "un-happy" who are most prone to speak up while the others who had good results go on about their business........ But I do vividly recall the adament and angry "consumers".....letting Aviation consumer know what they thought about the errent advice.

6. One other factor. Not all cylinders are made the same. Superior(the old original Superior from the late 1990's and early 2000's........BB......before bankrupcy) nitrited their cylinder walls differently than Continental and Lycoming. Such that it took longer to break in their cylinders. It took me and many others I knew at the time sometimes 100 hours to get oil consumption and temps to stablize. I shudder to think what might have happened had I went straight to AD oil.......

they employed a writer who was a fierce advocate

That wasn't Kas Thomas, was it?

(Must have been over 30 years ago, IIRC)

Marc,
I just got my C90-14F back from overhaul with Superior Millenium cylinders. Engine shop required mineral oil during break-in to be eligible for the warranty. So, in my case there was no temptation to try anything different.
Josh

Marc: What kind of cylinders do you have? (chrome, steel, cermi-nil, etc.)

6643 wrote: ↑Wed Oct 14, 2026 9:21 am Marc: What kind of cylinders do you have? (chrome, steel, cermi-nil, etc.)

Brand new Millenium cylinders from Superior. After reading through these posts and other sources, I'm going with straight mineral oil.

V529 wrote: ↑Tue Oct 13, 2026 9:12 pm From years in the business it seems as if Mineral oil is the best way to go for break in oil. It is and has been an industry standard for longer than I've been involved in the business. (1981)

I sure appreciate that detailed response. And after reading your post and several sources (like the ones you noted), and talking to a few IAs, I'm going with the industry standard straight mineral oil. Thanks again for the thought you put into your response.

just to add to the chorus...in 2024/25 i overhauled 3 chrome cylinders on my 120, one at a time, for various reasons. i too read online comments about how 20w50 was mineral based and worked for break in. I felt that since it was just one cylinder (at a time) that i didn't want the "hassle" of switcing to mineral oil like the Continental M0 manual says. My oil consumption during this time just continued to get worse and worse and blowby was the only explanation. when it finally dropped to a quart in 2.2 hrs, i gave up.

I went with 4 new steel millenium cylinders, ran mineral oil for 25 hrs per the manual and now us a quart every 20 hours.

Breaking in chrome cylinders is a whole other art form! That's why I asked what type we were discussing.

One thing that is as important as the oil is the engine load. The higher the better for break-in. But, you have to combine it with temperature control. New steel cylinders are rough, and generate a lot of friction. Get them too hot too soon and you can get tiny welds between the rings and the cylinder walls. The key is several short low power runs to knock off the high points, followed by cooling periods, followed by increasingly longer runs at increasingly higher power levels. After 2 or so hours run time, go fly for an hour at 80% power and high air speeds. (Don't spend the whole time climbing, you want lots of air flowing over the cylinders). Repeat this 2-3 times and you're probably about done. I'd still avoid a lot of low power operation for the first 10 hours or so. If you want to go out and watch cows, get someone else to take you.

Also, don't take this as a point by point process for breaking in your new cylinders!

6643 wrote: ↑Wed Oct 14, 2026 3:35 pm Breaking in chrome cylinders is a whole other art form! That's why I asked what type we were discussing.

One thing that is as important as the oil is the engine load. The higher the better for break-in. But, you have to combine it with temperature control. New steel cylinders are rough, and generate a lot of friction. Get them too hot too soon and you can get tiny welds between the rings and the cylinder walls. The key is several short low power runs to knock off the high points, followed by cooling periods, followed by increasingly longer runs at increasingly higher power levels. After 2 or so hours run time, go fly for an hour at 80% power and high air speeds. (Don't spend the whole time climbing, you want lots of air flowing over the cylinders). Repeat this 2-3 times and you're probably about done. I'd still avoid a lot of low power operation for the first 10 hours or so. If you want to go out and watch cows, get someone else to take you.

Also, don't take this as a point by point process for breaking in your new cylinders!

Thanks John. Thats very similar to what Continental says. Yours just explains it better