I used to know a lot about lubricants but I have forgotten most of it by
   now. I don't think that there is a standard measure for "film strength."
   Lubricants behave very differently depending on the loads they are
   subjected to. In a plain bearing, film thickness is highly dependent on
   tthe basic viscosity of the fluid. In applications with very high
   contact
   stresses, like rolling element bearings and gear meshes (and apex
   seals), film thickness is more dependent on the pressure-viscosity index
   (for which little data are published). In a highly loaded line contact
   bearing, the pressure on the lubricant entrained in the contact zone
   gets enormous. The viscosity increases dramatically along with the
   pressure. The characteristic of viscosity vs. pressure is different for
   different lubricants. The traditional viscosity and the
   pressure-viscosity index are strongly influenced by the base stock, less
   so by the additive packages. Another relevant measure is the shear
   stability. This is a measure of how much the viscosity changes under
   very high shear rates. I recall that the additives that increase the
   viscosity index (to make multi viscosity oils like 10W-40) are not very
   good under high shear rates. Finally, EP (extreme pressure) additives
   serve to protect highly loaded contacts like differential gears by
   providing a chemical film on the surface of the metals. These films
   provide some "cushion" and shear away preferentially when the fluid film
   is too thin to prevent metal to metal contact. The surface films prevent
   scuffing and adhesive wear. The API classifications (SF, SH, CF, etc.)
   require rigorous testing of many lubricant characteristics. I guess it
   is possible for an oil company to "cheat" and label a product falsely,
   but I doubt the reputable companies do this. Modern lubricants are very
   high-tech components of an engine. The reason automotive engines
   commonly run 200k miles and more these days has a lot to do with today's
   improved lubricants.
   Jon

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   Thanks for the helpful info Jon, it's getting close to the sort of
   stuff I have been looking for. I have taken the liberty of
   interposing comments with some of your remarks.
   I used to know a lot about lubricants but I have forgotten most of
   it by now. That's the trouble isn't it? Just when you've got things
   pretty well worked out you start forgetting it!
   Lubricants behave very differently depending on the loads they are
   subjected to. In a plain bearing, film thickness is highly dependent on
   the basic viscosity of the fluid. To me it seems that most of the
   bearings in a rotary are plain bearings, of course the gears are
   point or linear contact, as are the gears in the planetary set. When
   you look at the bottom end , or for that matter the piston pin
   bearing in a high performance 2 stroke you are looking at rollers in
   the big ends, balls for the mains, and possibly needle rollers in
   the upper end. Two stroke oils seem to be well formulated to look
   after these: -
    In applications with very high contact stresses, like rolling
   element bearings and gear meshes (and apex seals), film thickness is
   more dependent on the pressure-viscosity index.
   Please correct me if I have got it wrong, but I get the impression
   that a good 2 stroke oil would lubricate both the engine and
   reduction box rather well!
    The API classifications (SF, SH, CF, etc.) require rigorous testing
   of many lubricant characteristics. I guess it is possible for an oil
   company to "cheat" and label a product falsely,
   but I doubt the reputable companies do this.
   I was not inferring in any way that there was any "cheating" going
   on, but when you get into the weird and wonderful world of marketing
   minor differences are often used to justify quite incredible price
   differences. If it cost a lot it must be better - ask any woman!!
   So many things are bought to make ourselves "feel good" Hence the
   slogans " Things go better with Coke" and "Toyota! Oh what a feeling".
   So what's my point? Does Idemitsu Rotary engine oil do anything more
   than give you a warm inner glow, knowing you have allegedly bought
   the best there is, and is this relevant if you aren't anywhere near
   Le Mans? Where's the evidence that it's better in my engine than any
   of the usual high quality oils that cost a fraction of its price, or
   for that matter better than a good 2 stroke oil?
    John Evans.