Oil sender plug on the back of the engine block is another place for a leak. A very difficult place to see or work on too with a turbo sitting on top of it....
I personally like the two simpler oil leak locations, the one listed by ICFA, and the valve cover gaskets. In the case of a turbo, return and supply lines.... unless it's coming out the intake, and yes it can.
Quantities of oil will more than likely come from places of higher pressure(Not necessarily high pressure IE: 100's of psi- it obviously isn't spraying- veeerrry baaad), like the oil sender. I've seen several sender bibs(extensions) get cracked at the block because people either lean on them or from vibration.... like on older diesels...and gassers for that matter.
I wouldn't go head gasket first if you don't have oil and water mixed in either oil pan or radiator. Of course a simple pressure test will eliminate the heads.... or the radiator with a STANT tester. If there's water or oil where it shouldn't be inside.... don't run further or you could hydro-lock the pistons... worst case scenario is looking at the tops of pistons from the new viewing port created by water in cylinder.
Get that dye for sure... it will make things much easier. I'll be looking for some for my 671...
Thanks for that tip fellas.