All to do with the oils the dealers are expected to keep, and what the relevant manufacturer keeps Vauxhall 'super synthetic' was originally 5w40 specific to diesels, but they've now changed to a 5w30 suitable for petrols and diesels. If you were to look at the oil specs, you'll find that renault and vauxhall list similar oils as being suitable, just that they recommend whatever oil they normally use/sell through the dealers.