Right, an oil thread then….
Firstly, I do the same, i.e. buy Mobil1 0W-40 from Costco.
Secondly, all engine oils of the same grade can be mixed, it is en essential requirement that they do, even mineral with full-synth etc. However while no adverse chemical reaction will occur, using a ‘lesser’ oil obviously degrades the performance of the oil in the system.
Your options are to either fork-out for 1L of Mobil1 from Halfords or any Esso Petrol Station for around £13 (which is what I did when I last needed a top-up…), maybe cheaper from Opie Oils, or, if you really don’t want to pay that, get Halfords own-brand full-synth – Both my local Vx Parts Manager and Honest John claim that because Mobil makes both Esso Ultron and Mobil1 they are very similar oils, and apparently Halfords own-brand is Esso Ultron…
And last, where about are you? If you are local to me (London Kings Cross area) I have some left-over Mobil1 – Costco sell it in 4L containers and the V6 only takes 5.5L, don’t mind lending you a bit for top-up…