[Land-speed] Synthetic Motor oil

[Kirkwood]

I enjoyed the thread last week on oils and there was a lot of good
information. Thanks, Mayf for starting it and of course, thanks to all that
participated. The point of this reply is not to disparage any brand but to
bring up a point that was left out and might be worthy of your further
consideration.

Mobil 1 is highly regarded by many. Occasionally a superior product is
applied in a way it was not intended, resulting in unexpected or poor
results. About 10 years ago Mobil was having such success with its fully
synthetic product the Mobil Corporation decided to introduce it to the
general aviation community. At that time Aero Shell was about the only
engine oil used in piston engines. The usual claims were made about the
superiority of synthetics and Mobil 1 found its way into quite a few very
expensive engines (at that time Factory New was about $20K-$35k and
overhauls were about $15K-$25K depending). Within the space of a year the
Mobil 1 aviation community began experiencing cam failures and other
maladies. I can't remember the details but it was something about either the
copper in the bearings or something else leaching out due to the oil
chemistry. Lawsuits prevailed and Mobil ending up repairing slightly less
than 1,000 motors to the cost of a few million dollars! The product was
withdrawn from aviation and AFAIK it has not returned. 

So what is the point? Well the past thread discussed using synthetic motor
oil in transmissions or rear ends. But an important point was missed.
Bearing wear in the engine depends not only on the oil chemistry but the
hydrodynamic wedge created by bearing clearance and viscosity. Sure there
are other issues but to simplify lets skip to my point. The hypoid gear in
your rear-end works by SLIDING CONTACT. The centerline of the pinion is
below the centerline of the ring gear and as the gears engage they slide
against each other. The action is even different from spur or helical gears
in the trans. Gear oils are formulated for the friction of a hypoid gear and
seem to work well. Using any synthetic simply because it has superior
properties in a different environment may not be in your best interest.