An excerpt from some Amsoil literature:
Hard-working diesel engines present a serious challenge to the lubricants Tight clearances and intense pressures can generate enough force to tear apart the molecular structure of the oil, causing permanent viscosity loss. Permanent viscosity loss is termed “shear” and leads to accelerated equipment wear, oil consumption and deposit formation.
Shear stability measures a lubricant’s ability to withstand shearing forces without degrading to a lower viscosity.
To meet CJ-4 requirements for shear stability, the American Petroleum Institute (API) requires diesel oils to pass the Kurt Orbahn 90-Cycle Shear Stability Test.
Resisting shear and maintaining protective viscosity in the harsh operating conditions of diesel engines is challenge enough for many diesel oils, but maintaining viscosity in the face of fuel dilution is another challenge
altogether. Factors such as frequent starts, excessive engine idling, short trips and cold weather have contributed to moderate levels of fuel dilution in diesel applications for years, while recent issues with emission systems
have brought the fuel dilution problem to a whole new level.
For example, AMSOIL has documented increasing fuel dilution levels in 2007-2009 Caterpillar C13 and C15 onhighway engines. There are many possible causes, including problems with a unit injector or leaking seals. Another cause of fuel dilution is new emission systems using incylinder post-fuel injection, a process most 2007-2010 light-duty GM, Ford and Dodge diesel pickups use to regenerate the diesel particulate filter.
Because diesel fuel is a natural solvent, it causes a multitude of problems when it contaminates the oil, including reduced oil viscosity, reduced oil film strength, increased engine wear (particularly in the cylinder/ring area), increased volatility, weakened lubricant detergency, accelerated lubricant oxidation, varnish formation, acid formation/corrosion and low oil pressure.
The most notable problem associated with increased fuel contamination is reduced viscosity and the corresponding effect it has on oil performance. When combined with shearing conditions, as little as 4 percent fuel dilution is
generally enough to reduce an oil’s viscosity to less than the specified viscosity grade.
AMSOIL sent five competitive synthetic CJ-4 5W-40 diesel oils to an independent laboratory for shear stability testing. Knowing the tough environment that diesels present to lubricating oils, AMSOIL doubled the standard Kurt Orbahn 90-cycle test and had the oils tested for 180 cycles. Samples were then contaminated with 2 and 4 percent ultra low sulfur diesel (ULSD) fuel.
As the graph shows, even after being shear tested for twice the industry standard and contaminated with 4 percent fuel dilution, AMSOIL maintained viscosity and was the only oil to stay within an SAE 40 viscosity rating. As other oils lost viscosity due to shearing forces and fuel dilution, their ability to protect against wear was jeopardized.
Link to a PDF Look at the chart on page 3. 
If I don't hear other wise in the next month and a half, I guess I'll have to pony up the cash for the higher price.:mad2: I did read somewhere that someone was running the 10w-30 in Cali for towing in the desert. 
