This might sound like basic advice, but you would be stunned by how often it happens in the field, especially on machines that have been worked on by general mechanics rather than specialized heavy equipment techs. An excavator has three distinct fluid systems: the hydraulic system, the engine oil system, and the gear oil systems for the swing planetary and the two travel planetary gearboxes.
Sometimes, a mechanic will see a low fluid level on the travel gearbox dipstick and, not having the correct 80W-90 gear oil on hand, will top it off with 15W-40 engine oil to get the machine through the end of the shift. This is a death sentence for the gearbox.
Engine oil and gear oil have completely different additive packages. Gearboxes operate with sliding friction-the teeth of the gears are literally sliding against each other under massive shock loads. To prevent the metal from tearing itself apart, gear oil contains high levels of extreme pressure (EP) additives, usually sulfur-phosphorus compounds. Engine oil does not have these additives in high concentrations. If you run a travel gearbox on engine oil, the gears will quickly overheat, and the metal surfaces will micro-weld to each other and tear away-a process known as galling.
If you are buying a used excavator, pull the travel motor dipsticks and look at the oil. If the oil looks milky, it has water in it, which is bad. But if the oil looks thick, gray, and has a silvery, metallic glitter in it-like metallic paint-that is the signature of galled gears caused by running the wrong oil. Once a gearbox has that silver glitter in the oil, the hardening on the gear teeth is gone. It's only a matter of hours before the teeth shear off completely, leaving the machine stranded. Always carry the exact specified gear oil, and never substitute it with hydraulic oil or engine oil.
