The oil in a vacuum pump is the lifeblood of the pump. It provides the seal that enables the pump to create a vacuum. Because of this, proper maintenance - including routine oil changes - is critical for good vacuum operation.
Assuming that the pump is in good operating mechanical condition, you can easily determine if the oil is due for a change based on its color (see Color Chart below). If the oil is dark brown in color, or worse, like black coffee, it should be replaced immediately. The color chart below indicates the degree of thermal ageing & therefore the remaining useful life of the oil in your rotary pump.
Oil should be changed before it has reached color 4. If the oil is allowed to degrade to the extent that colors 6 & 7 are reached, the pump may have suffered damage & should be fully serviced prior to re-use. Good vacuum pump oil is, in general, 8 to 10 times the cost of typical motor oil, but is vital to proper pump operation. Oil used in rotary vacuum pumps is a special product, vastly different from motor oil. There are no additives such as rust inhibitors since such additives would be separated from the base fluid and adhere to internal pump surfaces as a gummy substance.