I respectfully disagree astro.
The ability for the fluid to not lose it's viscosity ( aka shear which is a byproduct of oxidation-amongst other components ) is interlinked with the chemical components that create the viscosity.
All things being equal...That is to say that a 75-140 will remain more resistant to oxidation than 75w-90.
http://www.machinerylubrication.com/...ture-stability
"Fluid exposed to high temperature can experience permanent deterioration. For example, a substantial reduction in fluid viscosity normally accompanies asperity contacts (mechanical rubbing) and an increase in temperature. In addition, irreversible viscosity change can also occur when a fluid having poor shear stability encounters high temperature. Whether through rapid oil oxidation promoted by high temperature with its accompanying sludge formation production, or simply accelerated component wear, the influence of high temperature on oil properties is serious and generally deserves prompt consideration and attention"
"The oil might lose its ability to lubricate if its viscosity changes significantly or if it is oxidized. The manufacturing tolerance on viscosity is plus or minus 10 percent. Therefore, ISO VG 320 oil should have a viscosity that falls within the range of 288 to 352 centistokes at 40 degrees C.
There are many possible causes for an increase or decrease in viscosity. For example, some oils have additives known as viscosity-index (VI) improvers that might not be shear stable. With time in service, these oils lose viscosity because the VI improvers shear down"
http://www.machinerylubrication.com/...-gear-failures
In addition, overheating might cause oxidation. Contamination by water and wear debris accelerates oxidation. The following symptoms are indicative of oxidation"
Given the location of the angle gear and it's small capacity for oil...Plus Ford's spec of 75w-140 in their cross-overs that use an angle gear...I don't feel uncomfortable using 75w-140.
Considering the failure mode of the Angle Gear and it's proximaty to the catalytic converter...I would rather try a preventive/ productive approach than wait for the common Angle Gear "reseal".
OEM's are under enormous pressure to meet MPG guidelines set fourth by the EPA...it's not uncommon for auto manufacturers to make lube choices at the expense of long term durability. Aka Striebeck curve-engine under full load results in increased wear when utilizing many an OEM's oil recommendation.
"Going heavier in grade isn't always "better". This is effectively splash lubricated. So, thinner might be better given the size and load on this assembly"
I agree with this statement, though in this application and design; I feel it's the most effective solution from a preventive standpoint.
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