Welch DuoSeal Vacuum Pump Hydrocarbon Vacuum Pump Oil, 5 Gallon 1407K-20. Welch DuoSeal Belt Drive Vacuum Pump Oil is fractionated especially for use in Welch DuoSeal vacuum pumps. DuoSeal Oil is checked to insure it meets rigid requirements for proper vapor pressure, chemical stability, viscosity and other essential characteristics. Long experience has proven the wisdom of using it. We recommend that you change your vacuum pump oil at least every 3 months to prolong the life of your vacuum pump.