If you have a full tank of fuel, you will have a positive head on the fuel pump, and oil will leak out if there is a problem there......if there were a problem in the area of the FI pump I, personally, would suspect that a reverse of the symptoms that were stated......

Supposing that there is a clog in the fuel tank vent......then with a full tank there would be a lesser volume of atmosphere within the tank.....thus causing fuel to flow more slowly.....when there is a greater volume of atmosphere within the tank (less fuel) then I would think that fuel would flow more easily...initially.....as it would take longer to pull a vacuum within the tank.....

Try removing the fuel cap when you have a full tank.....and see if it makes a difference.....if it does, as I suspect it will.....clean the tank vent and you will be good to go wherever and whenever....regardless of the amount of fuel within the tank.....

SB

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Diesels:
'85 300D, "Max, Blue Benz", 155K, 27.0 MPG
'84 190D 2.2, "Eva, Brown Benz", 142K, 40.2 MPG
'77 240D (parts car)
'67 Eicher ES 202 Tractor "Otto" (2cyl, Air Cooled, 30HP)
Gassers:
'94 Ford F-150, "Henry", 170K (300 Six) 17.5 MPG
'85 190E 2.3, 148K....Parts Car
'58 Dodge W300M Powerwagon (Flat Fenders) Less than 10 MPG