Excavator hydraulic systems require highly specific anti-wear (AW) hydraulic oils. A massive, growing problem in the field occurs when a mechanic tops off a machine running conventional zinc-based AW oil with ashless (zinc-free) hydraulic oil, or vice versa.
Zinc-based oils use ZDDP (zinc dialkyldithiophosphate) as an anti-wear agent. Ashless oils use sulfur-phosphorus chemistry. If you mix these two, the different additive packages react chemically. They precipitate out of the oil base, forming a thick, gray, sticky sludge that looks like wet cement.
This sludge quickly defeats the hydraulic system. It plugs the 10-micron return filters, starving the charge pump. But worse, it migrates into the main control valve, packing the tiny orifices in the load-sensing network. The pump will suddenly go on full stroke and won't destroke, blowing the main relief valve at 5,000 PSI and generating massive heat. If you ever see a gray, gelatinous paste on the hydraulic filter magnet, the system is contaminated with incompatible oil. The only fix is to drain the 100-gallon tank, flush the lines with kerosene, and replace the oil completely. Never mix AW and ashless fluids.
