You think you can try to run the car on a spare bottle ( in close circuit, fed and return on the same hole)under the hood to see if the pump actually pump, this after changing the filter and cleaning the prefilter to be sure all is clean there.
A one litre can get you about 5 milres under heavy revs/ loads.

__________________
E300TD year 2000. RUSTY SOLD
cost a fortune to maintain on the road
but run well on WVO
Second Merc died due to corrosion ( NOT rust) How can mercedes get away with that for so long?
Third lasted a month then went away...
Fourth now... Corroded too...