Start using the dipstick it might have been well below the minimum point on it when you added a litre you probably could of added more

Since changing from castrol to millers iv noticed a considerable drop in oil consumption over 18 months still likes a drop just not as much i dont keep records though just got used to it and always keep a litre in the boot

Do your ccv if its still the same try moving to a 10w40 oil

A quick simple fix for m54 oil consumption doesn't exist unfortunately