It could be a couple things, the rear main seal, the turbo oil supply line could be leaking, or the drain tube/pedestal could be leaking where it drains into the valley. The oil line on mine was leaking for a long time I thought it was the rear main seal. When I rebuilt my turbo I was told to take a punch and poke a hole on the inside of the baffle in the valley because it won't always drain fast enough for the oil draining from the turbo, causing the an oil overflow into the valley and down the back of the engine. IDK if poking a hole was the right thing to do but it seemed logical to me.
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