Samiam4,

The engine oil lubricates only the bottom of the injection pump: the cam , bearings and lifters. Its the barrels and plungers that the fuel lubricates. The clearance between the barrel and plunger is smaller than the molecules of fuel so there is no leakage between the plunger and barrel but there still needs to be some lubrication since there is movement and metal to metal contact would cause wear.

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