The OP said that he lost eight quarts of engine oil over 160 miles (that is a 3-4h drive). That makes it unlikely that it is gear oil from the transmission.
The concern about fuel getting into the oil is something to check, though, before dropping the tranny etc. When engine oil becomes diluted by fuel, seals will start to leak. The oil will appear thin and fuel-like on the dipstick, it may or may not smell strongly of Diesel; you can always get a test kit, take a sample and send it off. Easier than trying to investigate FDC, flame heater etc. without a clear smoking gun.
If the oil turns out to be ok and it is indeed the rear main seal, it is a good time to think about what else to do and what to order when you drop the transmission and take the flywheel off. New clutch and throwout bearing, resurfacing flywheel? You probaby won't enjoy wrestling with the tranny enough for wanting to do it again anytime soon.
In any case, buy a brand new production seal (NO "OEM" old stock stuff) and also check the seal carrier for the oil return hole modification (if not done, drill them ).
