If your Fuel Supply/Lift Pump is the same as the 300D ones on the MW Fuel Injection Pumps there is only 1 O-ring Seal on the whole Lift Pump.
When that O-ring goes bad it is more normal for the Fuel to come out of the Lift Pump and mix with the Engine Oil.
However, 2 members have said a bad O-ring there sucked Oil out of the Fuel Injection Pump and mixed the Oil with the Fuel that was burned up in the Engine; there symptom was their Oil was disappearing but they had no Oil leaks and there Fuel was turning blackish from the Oil.
In this thread you can see part of the Lift Pump Taken apart (Post #9) and there is a tiny O-ring. The size of the ring is given in the thread (Posts #11 and #14).
http://www.peachparts.com/shopforum/showthread.php?t=266857&highlight=lift+pump+kit.
However, this is just a regular O-ring. It is not a Viton O-ring.
However, you might want to post in the biofuel section of the forum and see if a regular O-ring in the Lift Pump causes a problem with other Fuels.