One gallon of oil is just about the right amount. For the gray, took the trap loose under the bathroom sink, emptied out the water, reconnected it and poured the vegetable oil down it. Then when done with gray valve, removed oil from trap and used it down the toilet for the black tank valve. For the black tank, just pour down the toilet. After getting the oil in the tanks, work the valves open an closed several times to work the oil into the gate seals until they slide easily. On ours, it took putting about 1/2 gallon of oil in each tank and running it through twice to get them loosened up real good. Did the gray first and then reused it for the black tank and then used fresh oil the second time through for the black. One gallon of cheap vegetable oil (fry oil) at GFS is about $4.50.
Rick and Janice Binns
2006 MS 32TK3, Mor/Ryde Pin Box and I.S., Auto Level-Up
2012 Ford F450 6.7 L Diesel, air bags and 25K Reese Signature hitch (factory installed)