Consider what happens when Eric, an electrician, replaces a failed motor. Being diligent, he checks the gearbox and finds it's low on oil. He fills it with 140W gear oil from a drum in the maintenance shop, and also greases a suspect bearing.
Because Eric used grease that's incompatible with the grease already in the bearing, that bearing will soon fail. Because this gearbox requires 90W gear oil and he used a much heavier oil, several problems including increased motor current will result.
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