the delay happen in the morning only (when the gearbox is cold ), once ur car become hot it will be fine ... gearbox need's replace, oil or filter will do nothing sorry to till you that ... or you can replace the clutches (valve body) iam not sure of its name but it needs an expert on auto trans coz bmw gear box is too Sensitive
take care ur gearbox will die withen one month try not to accelerate to hard when the car is cold