Stand on the drivers side of the car. Lean over and look down between the back of the engine and the brake booster. You should see a round, black cap. You'll need an oil filter wrench to remove this cap. It is reasonably priced at the dealer. Remove the cap and replace the filter element. Not sure what the official capacity is - I think it was changed. If you put in 7 quarts it will be between the lines on the dipstick.
1984 300D Euro