I just called the customer service dept. on 0808 100 0015, and they were very helpful for me.
Yes, don't accept nonsense about 'using old stock'. If they are advertising Castrol Edge they have to supply it. If this is a widespread practise then Trading Standards should hear about it.
If you get 'what it says on the tin' it's a good offer, if not it's literally too good to be true.