Generally excessive pressures can cause pitting to increase in alot of cases, Thicker oils tightens tolerances etc...I'm no expert but a car gearbox gets jerked back and forth alot which you'd think a thicker oil would stop alot of shock value in the backlash. But at the same time, The thicker oil, wouldn't create a higher pressure between the gears creating an increase in pitting/wear etc..At the end of the day, the harder you smash gears, the more power you place through a box the higher the pressure, the more wear anyway. Nothing stops age and wear and tear destroying a box, It's self distructing from day 1 as the case hardening slowly wears through into the gears. Most of the boxes we use, the damage has been done.
I hope that made any sense at all