Marvel Mystery Oil

[PT20J]

A couple of years ago, I noticed the compression on one cylinder on my Lycoming IO-360-A3B6 was trending downward on successive measurements from 78 to 74 to 68. I added Marvel Mystery Oil to the fuel for a couple of tanks and the next time I checked the compression, it was 76. Might have been a coincidence. 

Recently, I had a cylinder drop out immediately after cold engine start. It happened again the next day. Maybe a valve beginning to stick? I added Marvel Mystery Oil to the fuel for one tank full and haven't had the problem since. Again, it might have been a coincidence.

A local mechanic swears that it makes a great ring flush.

Supposedly a lot of DC-3 operators used it to prevent valve sticking in R-1830s back in the day. The mechanic at a museum where I used to volunteer told me that back when the FAA operated several DC-3s as flight check airplanes a mechanic that serviced them told him that it was used it in these planes for the same reason. Apparently they bought it it 55 gallon drums.

What's in the stuff? I've attached the SDS. It's Petroleum distillates, hydrotreated light (solvent), Methyl salicylate (wintergreen fragrance), TCP, and two isomers of Dichlorobenzene. TCP is a known lead scavenger. Some searching shows that dichlorobenzene might soften carbon and varnish deposits. So, theoretically it might be beneficial. I don't know, but I'm curious what others have experienced.

MM12R_50094_MM13R_50095_MM13RC_Marvel_Mystery_Oil_SDS_US_031424_FINAL.pdf

[N201MKTurbo]

Never in an airplane, but I have used it successfully in automobiles. A cup of mineral spirits will work just as good.

[varlajo]

I use it all the time. 1/2 quart into the oil for a 1-hour flight before every oil change. 

[PeteMc]

1 hour ago, varlajo said:

for a 1-hour flight before every oil change. 

I've never heard of using it just before and oil change.  Those I know that use it add it with new oil... at least in a Contenental.  I guess there can be some reasons not to add it to a Lycolming. 

 

[takair]

I didn’t vote since I don’t use it in the Mooney but have used it in a small Continental with good results. A “splash” with each tank of fuel. I’ve heard many positive results and many neutral results, don’t think I ever heard of negative results. I do wonder how it might affect bladder tank or sealant, but again have not actually heard negatives. I’ve also read of folks adding it to oil prior to a change like the poster above. In that case I worry about loosening sludge if you have not used it since new. That said I can also imagine some of the benefits. Seems to have as many variable uses as Jose (Piloto) had for WD-40. 

[1980Mooney]

2 hours ago, takair said:

I didn’t vote since I don’t use it in the Mooney but have used it in a small Continental with good results. A “splash” with each tank of fuel. I’ve heard many positive results and many neutral results, don’t think I ever heard of negative results. I do wonder how it might affect bladder tank or sealant, but again have not actually heard negatives. I’ve also read of folks adding it to oil prior to a change like the poster above. In that case I worry about loosening sludge if you have not used it since new. That said I can also imagine some of the benefits. Seems to have as many variable uses as Jose (Piloto) had for WD-40. 

I also didn’t vote because my use didn’t fit the poll. But I did notice a negative result. 
 

There was a period about 10-15 years ago that I used it consistently on a Continental IO-550A every tankful, mixing per the instructions on the can. I noticed a couple things:

- It changed the color of the exhaust deposits in the tail pipe.
      * In variable throttle position shorter flights the deposits were less charcoal black and more a soft dark grey color. These are flights where you are generally running rich producing more carbon deposits.
     * On long flights where I leaned out, the typical light grey lead color deposits were slightly darker soft grey color.  

•• So it seems to change how the carbon forms and deposits. 

-  My electric fuel pump would run quieter.  

- On my wing walk, which is older weathered black, I could start to see discolored spots that were getting darker. There were screw head “halos” appearing.
 

I immediately stopped using it for fear of damage to the fuel tank sealant. 
 

BTW - I never was able to direct, measure or notice any change in engine performance or reliability before using, while using or after discontinuing use. 

[Paul Thomas]

I've never used it, but I've never been in a position where it's been recommended either.

[takair]

1 hour ago, 1980Mooney said:

I also didn’t vote because my use didn’t fit the poll. But I did notice a negative result. 
 

There was a period about 10-15 years ago that I used it consistently on a Continental IO-550A every tankful, mixing per the instructions on the can. I noticed a couple things:

- It changed the color of the exhaust deposits in the tail pipe.
      * In variable throttle position shorter flights the deposits were less charcoal black and more a soft dark grey color. These are flights where you are generally running rich producing more carbon deposits.
     * On long flights where I leaned out, the typical light grey lead color deposits were slightly darker soft grey color.  

•• So it seems to change how the carbon forms and deposits. 

-  My electric fuel pump would run quieter.  

- On my wing walk, which is older weathered black, I could start to see discolored spots that were getting darker. There were screw head “halos” appearing.
 

I immediately stopped using it for fear of damage to the fuel tank sealant. 
 

BTW - I never was able to direct, measure or notice any change in engine performance or reliability before using, while using or after discontinuing use. 

Thanks, interesting observations.  I will say, in the small Continental we don’t add aggressively…so not as much as instructed.  We also generally run it rich mixture. Very different engines for sure.  I do suspect it might alter the octane rating and fuel burn rate a little.  That might explain exhaust color.  Would be really interesting for someone with a test stand to do testing, but I suspect little motivation to spend the money…

[MooneyMitch]

The whole thing is a complete Mystery to me.....................I do like the way it smells...............................

[bonal]

Many years ago the fuel pumps in my 89 f250 started failing both front and rear tanks.  Put some MMO in the tanks and problem never came back.  Still to this day running just fine.

[EricJ]

I've used it, I've known a lot of people who've used it in aircraft in both fuel and oil.   Supposedly many 55 gallon drums were used during WWII in standard procedures to keep aircraft engines working.

An acquaintence who was a rep for an aviation oil company said using Aeroshell W100+ will prevent stuck valves, too, and probably unstick stuck ones.   

I don't think MMO would be an essentially legendary aviation supply for about a hundred years without being effective at something.

I've used it for ring flushes as well as in fuel.   One of the nice things with using it for a ring flush is it's not tragic if some of it gets left behind in the cylinder or in the sump.

 

[1980Mooney]

1 minute ago, EricJ said:

I've used it, I've known a lot of people who've used it in aircraft in both fuel and oil.   Supposedly many 55 gallon drums were used during WWII in standard procedures to keep aircraft engines working.

An acquaintence who was a rep for an aviation oil company said using Aeroshell W100+ will prevent stuck valves, too, and probably unstick stuck ones.   

I don't think MMO would be an essentially legendary aviation supply for about a hundred years without being effective at something.

I've used it for ring flushes as well as in fuel.   One of the nice things with using it for a ring flush is it's not tragic if some of it gets left behind in the cylinder or in the sump.

The only "fly in the ointment" is that if it is so great, so beneficial, then it would be documented in tests.  SAE, AIChE, Journal of Petroleum Science and Engineering, AIAA, SFTE, Internal Combustion Engine Division of ASME, Big Oil, DOD or its predecessors over 100 years, Lycoming, Continental, Franklin, Rotax....somebody, anybody would have tested and documented the benefits.  And if there were any it would be licensed and incorporated/blended into either AVGAS or aviation lubricants.  And if there were documented benefits there would be all sorts of knock-off competitors.

BUT - you can't find anything anywhere. NADA.  Only hangar talk.

[Fritz1]

lots of lore, to my best knowledge not approved by any aviation engine manufacturer, have used it in oil as a detergent 1h before oil change, oil definitely turns darker, what are the pros and cons, not sure, do the pros outweigh the cons, don't know and don't have any in my hangar, using Phillips XC which appears to be doing just fine with 25h oil changes in Bravo engine, MMO appears to do a good job as a ring flushing agent, residue burns off easily after flushing is done

[PT20J]

I think the only additive Lycoming approves is LW-16702 which is TPP.

[Jackk]

 

 

Not today Mr FAA lol

 I have heard from other people, if you use it as the bottle states it can help with seals and valves, worst case it doesn’t do anything.

 

 I think most people who run it are using it to add to mogas for the added lube

Edited August 24, 2025 by Jackk

[PeteMc]

15 hours ago, 1980Mooney said:

I immediately stopped using it for fear of damage to the fuel tank sealant. 

Just remembered that one guy I knew was using it in the fuel only added it just prior to a long XC.   He said that way he got all the the benifits of cleaning things out in the fuel system without having any left over when the plane was just sitting. 

 

[ArtVandelay]

Just remembered that one guy I knew was using it in the fuel only added it just prior to a long XC.   He said that way he got all the the benifits of cleaning things out in the fuel system without having any left over when the plane was just sitting. 
 

That’s why I add a 1/2 qt before my oil change preflight, but I still don’t want it in my fuel tanks.

[Pinecone]

A long time ago I instructed in a C-150.  This was in the days where 80-87 had just gone away and the O-200 was not happy on 100LL.

We just a bit of MMO in the fuel after having some valve sticking and it did not have issues again.

[76Srat]

Shhhhh . . . don't tell Cirrus.  They'll ban its use in their aircraft immediately, for "not compliant with the STC" . . .

[M20F]

Seafoam is better 

[201Steve]

1 hour ago, M20F said:

Seafoam is better 

I’ve wondered why it hasn’t been discussed more. It’s a staple in the auto world. Pretty similar, is a petroleum product you can put pretty much anywhere. 

[PT20J]

3 minutes ago, 201Steve said:

I’ve wondered why it hasn’t been discussed more. It’s a staple in the auto world. Pretty similar, is a petroleum product you can put pretty much anywhere. 

According to the SDS, it appears to merely be light oil, naphtha, and isopropyl alcohol. Probably a good degreaser and starting fluid, but I wouldn’t expect it to have an effect carbon or lead deposits.

[M20F]

14 minutes ago, PT20J said:

According to the SDS, it appears to merely be light oil, naphtha, and isopropyl alcohol. Probably a good degreaser and starting fluid, but I wouldn’t expect it to have an effect carbon or lead deposits.

I dumped it into a 2009 Outback filled with sludge in @2021.  It was leaking a bit of oil and thus I never changed it over the course of 4-5 yrs and maybe 20K miles.  
 
Driving from ORD to ATL it blew all the crap out and the car leaked like a sieve because of all the sludge being gone.  Mechanic put some gaskets and other junk in, said it was a really clean looking motor.  Still running though I try to change the oil at least once a year now (it’s down to maybe 1k miles a year now). 
 
Testament to Subaru and Seafoam in my book.  YMMV.  

[PT20J]

5 hours ago, M20F said:

I dumped it into a 2009 Outback filled with sludge in @2021.  It was leaking a bit of oil and thus I never changed it over the course of 4-5 yrs and maybe 20K miles.  
 
Driving from ORD to ATL it blew all the crap out and the car leaked like a sieve because of all the sludge being gone.  Mechanic put some gaskets and other junk in, said it was a really clean looking motor.  Still running though I try to change the oil at least once a year now (it’s down to maybe 1k miles a year now). 
 
Testament to Subaru and Seafoam in my book.  YMMV.  

Throwing in a pint of Stoddard Solvent would do the same thing. It’s an old shade tree mechanic trick for flushing out sludge. But generally, they only ran the engine for a short time before draining.

[PeteMc]

12 hours ago, M20F said:

Driving from ORD to ATL it blew all the crap out and the car leaked like a sieve because of all the sludge being gone.

My B-In-Law got his grandfather's old car when he was heading off to collage in the 60s.  Changed the oil in the car with some of that great modern degergent oil that was being advertised.  Got about 60 miles and the engine seized up.  When they eventually got it to a shop, the mechanic said it was because of all the crap that new detergent oil broke free and it got everywhere inside the engine.  

 

 

Edited September 1, 2025 by PeteMc