Did you check with the pump running if the O-ring was leaking ? You should be able to get away with the same size and a few wraps of teflon tape as well. I think the o-ring is usually pushed out if it has failed. Sometimes the lower part which is a plastic to plastic fitting can leak too which you could see with the pump running.

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1988 325is (purchased new) sold
2004 X5 3.0 2005 X3 2.5
2008 X5 3.0 (new to me)