Ran across a thread in a racing forum I'm on where a chemist was discussing the actual ingredients. It's basically a lightweight mineral oil (~3W) mixed with a solvent. The solvent was described elsewhere as "Stoddard Solvent is whats in WD-40, approx 70%. Its also the first item in Kingsford Charcoal Lighter, and is in most alkyd paints and stains. In fact its 90% of a certain deck stain."

MSDS:
http://www.setonresourcecenter.com/m...3/wcd04377.htm

There was also a link to an interesting newsletter on various additives in general from a company that performs oil assays. In the solvent section (where MMO is listed) they conclude it's "moderately" useful in 50's engines, and unnecessary in modern engines.

"In general, these products are designed to do just the opposite of what the PTFE and zinc phosphate additives claim to do. Instead of leaving behind a "coating" or a "plating" on your engine surfaces, they are designed to strip away such things.

All of these products will strip sludge and deposits out and clean up your engine, particularly if it is an older, abused one . The problem is, unless you have some way of determining just how much is needed to remove your deposits without going any further, such solvents also can strip away the boundary lubrication layer provided by your oil. Overuse of solvents is an easy trap to fall into, and one which can promote harmful metal-to-metal contact within your engine."
It's a PDF download but it's an interesting read overall:

http://www.blackstone-labs.com/newsletters.php?ID=30

Thought some folks might like knowing what's actually in it and what it does.