On a cold?climate excavator, you might check the hydraulic tank and see a label saying either AW 46 or HV 46. Both are 46 weight oils, but at –10°C their behavior is completely different. Mixing them is a fatal mistake.
AW (anti?wear) hydraulic oil is standard, low?cost oil. It uses a single base oil. In summer it's fine, but below freezing AW oil turns to syrup. HV (high viscosity index) hydraulic oil uses synthetic or severely refined base oil plus viscosity index improvers-long?chain polymers. These curl up when cold, letting the oil flow, and uncurl when hot to maintain thickness.
In winter, if a mechanic tops off an HV system with cheap AW oil because "it's all 46," those long?chain polymers can be destroyed. The blend loses its high?VI properties. Next morning, when the operator starts the machine and snaps the joystick, the thick, blended oil simply won't fill the main pump. The pump cavitates, screaming, and tears microscopic metal particles from the pistons and valve plate, causing thousands of dollars in instant damage. Never mix AW and HV oils in machines that work in winter, and be wary of suppliers who substitute cheap AW for HV to save a few dollars. The damage from cavitation far exceeds the savings on oil.
