There is one additive, however, that got a stamp of approval. Back in the
late 1970s, Textron Lycoming had problems with a new 320 cu. in. engine
fitted to the Cessna 172. They had replaced the camshafts and tappets with
narrower lobe camshafts and automotive lifters, and they started having
camshaft and lifter spalling problems. The Federal Aviation Authority (FAA
)issued an Airworthiness Directive that said owners had to put a special
additive, LW-16702, in their O-320H2AD engines. It's about $11.00 for six
ounces, which is enough for the 6-quart O-320 crankcase. You have to run a
lot of tests and prove to the FAA that a substance won't do any harm before
you can get their approval to put it in an airplane. To be forced to put
some after-market stuff in your airplane means it has to solve a problem.
If anybody's interested, I can find out the active ingredient; it's not a
secret. Shell 15W-50 aviation oil (semi-synthetic) has the additive already
in it.
There is another additive, called Marvel Mystery Oil, that is of the more
usual snake-oil variety, and a lot of aviation enthusiasts use it. It's not
approved by the
FAA, however. Opinions on its effectiveness are mixed.
By the way, if you really want a good one, I just got a J.C. Whitney catalog
that lists a "magnetic fuel saver" for $15.95 (regular $19.95!) that delivers
magnetic pulses to your fuel line and "breaks up clustered fuel molecules" so
you get a more complete fuel burn and an 11.6% increase in fuel economy (EPA
accepted laboratory tests!!) Well, it'll probably be supressed by the oil
companies, just like the old 100 mpg Pogue carburetor was in the 1930s. :-)
Egon