BaileyrockAgreed, but these "C" category oils have extremely high anti sheer characteristics that should work well in or gear boxes, No?
True but even if the polymers shear they still work to thicken the oil...
Polymers are exposed to two major degradation processes in the engine:
mechanical or shear degradation and thermo oxidative degradation. All
polymer-thickened oils experience a loss of viscosity at shear rates
corresponding to those existing in engine bearings. As the coiled
molecule enters a zone of high velocity gradients, it deforms. This
lengthening of the molecule allows more base oil to flow past and
the oil under high gradient shear loads experiences a viscosity loss.
Upon passing through the velocity gradient, the molecule regains
its former coiled state and the viscosity of the oil recovers. All
other things being equal, the higher the molecular weight of the
polymer molecule, the more temporary shear loss will occur.
Complete alignment of the molecule in the shear zone cannot occur
because of the counteracting entropic forces that act to retain the
coiled shape; i.e., the molecule tends toward disorder. Under severe
operating conditions such as often exist at the entrance to gear
teeth, it is possible the shearing forces acting on the molecules may
exceed the bond energy of the main chain. If the molecule is unable to
deform, the chain tears apart. This shortens the average size of the
polymer molecules and results in a permanent lowering of oil
viscosity. High molecular weight molecules (long molecules) and
molecules with weak chains have the greatest amount of tearing.
The degree of viscosity loss depends on the type of VI polymer
molecule used. Some molecules such as the polyalkylmethacrylates have
strong chains. The olefin copolymers have weaker chains but the broken
chains react with the oil and have a thickening effect. Some polymer
molecules are highly branched or incorporate dispersant side chains
which effect the polymer's ability to tolerate shear forces. The
additive engineer must therefore select a polymer type and molecular
weight that fulfills the intended purpose. Modern VI-improved oils are
adequately shear stable for aircraft engines. Straight-weight oils do
not suffer temporary viscosity loss with shear as multi-grade oils do
The viscosity of multi-grade oil in the engine will not exactly
correspond to the viscosity of the oil in the can. A lOW50 oil does
not have the same viscosity as a 50 weight oil under engine (shear)
conditions. At the oil pump, yes, but at the journal bearings where
shear forces are five or six orders of magnitude higher, the oil will
have the viscosity of a 40 weight oil. This might be more noticeable
during cold cranking where a lOW-50 multigrade may cold crank better
than a lOW oil due to temporary shear viscosity loss. Oil viscosity
does not accurately predict cold pumpability.
Thermal oxidation degradation of the polymer molecule is much more a
problem than shear degradation. During shear degradation the longer
molecules are sheared so the average molecular weight of the molecules
becomes more homogeneous. Oil viscosity loss stabilizes at this point.
With thermo-oxidative processes there is no limiting molecular weight.
The degradation process continues until the polymer is completely
destroyed. This process has a thinning effect on the oil. At the same
time, the thermo-oxidation of the oil results in a viscosity increase
of the base oil. Regular oil changes, especially under severe
thermo-oxidation conditions (turbocharging) is important.
Any decrease in oil viscosity increases wear rates and oil
consumption. If you are changing your oil at 50 hours and you notice
that your oil consumption is less during the first 25 hours than the
second 25 hours, you know that this oil is only viscosity-stable for
25 hours. You might consider shortening your oil change interval or
switch to a different brand of oil.