With proper pump maintenance you can get a continued return on your pump investment . Change the oil after every use while the oil is still hot.
Contaminants are still in suspension and are removed with the oil.
If contaminants cool, solidify and stay in the pump, they lower vacuum efficiency. In extreme cases, the oil stops lubricating and the pump seizes. Oil color cannot tell you if the oil is contaminated. The only way to determine oil condition is to test vacuum pulled with an electronic vacuum gauge. When finished with the pump, replace the tethered caps on the intake fittings.