Its normal for an M54 to consume a litre of oil per 1000 miles so get used to keeping a 1.0 bottle in the boot

I run 5w30 fully synth in mine (dont buy ford spec) for what its worth i swapped from castrol to millers and noticed a considerable drop in consumption although it was never really that high

One common cause is the ccv

Top it asap though you dont want her running dry i dip mine every forthnight now out of habit (yes i know what i siad )

Dont wait for the light of doom to come on