Its just engine oil. I believe the capacity is something like 0.7l. On all except the early diesels the IP oil is fed in through the small line on the outboard side of the pump and exits in the vicinity of the input shaft into the timing chain area. However, it is necessary to fill the IP after you drain its oil because the flow rate through the small feed hose is low.

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99 E300 Turbodiesel 100k