Both Marvel Mystery Oil and Avblend are intended for much the same purpose; a reduction in valve sticking and reduced engine wear.
Many old-line mechanics swear by Marvel Mystery Oil. All the evidence I have ever seen, asserting its effectiveness, has been anecdotal. I have seen no published study associated with aircraft engines that validates the claims. MMO has not been approved by the FAA or any oil manufacturer for use in certified engines. It is relatively inexpensive and is readily available.
Avblend received FAA approval based on an engine teardown and published results. Minimal wear was demonstrated. The FAA approval means that the FAA agrees it will not harm the engine. It does not mean that the FAA agrees that it will provide all the claimed benefits. It is relatively expensive, and is usually ordered by mail.
The engine manufacturers and oil manufacturers do not like aftermarket oil additives, with the sole exception of the Lycoming anti-wear additive. That additive is now incorporated in the Aeroshell Plus oil, the Phillips 66 AW oil, and probably others.
Keep in mind that aircraft engines are quite different from auto engines. Generally speaking, aircraft engines have much larger running clearances; must deal with larger changes in those clearances during the range of operation; must deal with a much wider range of temperatures and higher localized temperatures; and operate at a far higher average percentage of maximum output. Few car engines run at an average load exceeding about 10% of max power; few aircraft engines average less than 65%. Any auto additives that contain metallic compounds can create ash in the cylinder, leading to preignition or detonation. Most auto oil additives, as well as automotive oils, contain metallic compounds.
My personal opinion is that, aside from legalities, it is an unwarranted risk to use MMO (and similar products) in an aircraft engine. It may work fine in an O320 operated at 60%, and may not work fine in a IO360 operated at 85%. In either case, you'll never hear anything but the anecdotal positive stories. Would you advertise having used an illegal additive, when you had to have your engine topped/cylinders repaired at 800 hours? If you are determined to use an additive, I would use Avblend.
And finally, my opinion is that insuring you have no hot-spots in your cylinder heads (occluded fins/excess casting flash between the ports), insuring that you have well sealed baffles, and changing the oil at 20 hours instead of 25, will cost no more. These steps will have a very definite positive impact on cylinder and engine life, as opposed to the indefinable benefits of oil additives (approved or otherwise). Note that this opinion excludes the required Lycoming additive (in certain engines); you must use it when it is specified, and it is probably a worthwhile anti-wear addition to the oils that incorporate it.
For additional reading and education, search the MML Archives for Bob Steward's findings on excess cylinder fin flash and its correlation with sticking valves. There is also additional information on BAC concerning oil selection and oil changes. If you have access to BAC, the MML searches will be far easier on that site.
This note will be added to BAC as an FAQ, as well as an imported MML note.
----- Original Message -----
From: Jeff Ryndak
Sent: Saturday, September 10, 2005 9:47 PM
Subject: [musketeermail] Oil additives
My mechanic that did my annual last week suggested I use Avblend. When
I went to buy some another mechanic said buy Marvel oil additive. Is
any one any better than the other and should I bother at all. I real
reply would be of big help. I did a search and did not come up with
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