These two photos show the
that both oil and no oil can do.
The above left photo is the result of a Ford 9"
differential being filled with the wrong oil. Wrong oil in this
case being an oil that has no "extreme pressure" additives to
accommodate the surface pressures during contact under load. The
gear-set will just wear away till you could nearly shave
yourself with the pinion teeth.
In the right-hand photo, the Toyota Hi-Lux pinion is on
it's way to becoming a molten blob. The differential has run out
of oil, but the driver has not realized it and has kept on
driving. With no oil in the differential, there will be no heat
dissipation and consequently the differential internal
temperature will continue to rise. As the temperature rises, the
metal will become softer and parts will begin to weld to each
other, thus eliminating component co-operation. Complete seizure
will be the result.