Garret T3 oil return leak

[2strokesmoke]

Hi guys,
I have a question reguarding my garret T3 oil return leak.
)engine setup id from an 85 Quantum TD )
I rebuilt turbo upon install of engine,gut have an oil leak when engine is warm and oil thins out.
It is leaking between turbo cartrige or center section and the 2 bolt flange where oil return line connects to turbo.
I had ordered a new gasket for this,when rebuilding turbo-but upon disassembly I found a rubber packing which sits in a groove,(similiar to an "O" ring situp)instead of a gasket.
The packing looked to be in great condition and was very pliable,so I reused it.
Well- upon first startup and engine warmup- the packing is leaking.
My question is : Should I replace with a new packing (dealer part-if still available) or should I use the gasket ?
Which is better ?
So far, I haven't found a listing for the packing - but plenty of listings for a gasket in this location.
Thanks for your help, Mike