While you are doing an oil service, check the small O-rings on the oil filter stem pipe (that runs down the center of the filter element) the two near the bottom and these tend cause a reduction in oil pressure at idle if they dry out (and crack).

The pipe is hollow and should be blown out with compressed air also, to be sure its not plugged.

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'95 E320 Wagon my favorite road car. '99 E300D wolf in sheeps body, '87 300D Sportline suspension, '79 300TD w/ 617.952 engine at 367,750 and counting!