The failure mechanism
If viscosity is too low for the load and temperature, film thickness may be insufficient and gear teeth or bearings can move toward mixed or boundary lubrication. If viscosity is unnecessarily high, churning and friction losses can increase temperature and energy use.
Micropitting and surface protection
Modern hardened gears can be sensitive to micropitting under high load. Lubricant formulation, viscosity and EP/antiwear performance all matter, so the gearbox OEM recommendation should lead selection.
Procurement implication
ISO VG 150, 220 or 320 are not interchangeable labels. Confirm operating temperature, load, speed and the OEM or gearbox-builder specification before quotation.
This article is general technical information and is not a substitute for the equipment OEM manual, lubricant manufacturer recommendation or site-specific engineering review.