Imagine there's a new technology that can save consumers money, at no expense to them. It sounds like a perfect PR story. But that's to ignore people's inertia, the media's suspicion of a PR-generated story, and the difficulty of grabbing attention when people are selective about the messages they're willing to receive.
There's a randomness to our message and media consumption. Here are some of the factors that worked for me in the Shell Fuel Save campaign:
Availability and attention: I picked up a discarded Metro newspaper on a commuter train last week. Wrapped around the news content was a prominent advertisement for the improved fuels. I had the time; I had the interest (self-interest), I read the message. I live remotely so I'm dependent on my own transport, and a Shell petrol station is nearest to and most convenient for me too.
Warm-up and follow-through: Only now that my interest was gained did I recall reading something about 'Freddie' Flintoff driving further on one litre of fuel. Later, when I filled up at the Shell petrol station, the staff were handing out leaflets explaining the fuels. There were prominent signs to ensure the attention of passing motorists too.
Word of mouth: It may be because of my age and income, it may be because of environmental concerns too, but I'm much more interested in a car's economy than I am by its acceleration. So I do keep a close eye on my fuel consumption and the early indications from my two diesel-engined vehicles are that the new fuels are noticeably more economical.
Some other factors worth mentioning in this short, personal, account of campaign effectiveness are:
Context: We don't like the major oil companies – but with BP suffering damage to its reputation this year, now is a good time for Shell to gain advantage through a positive campaign.
Multi-media: In major campaigns (this one is global), it's never a case of PR or advertising; marketing or graphic design. It takes a combination of media channels for the message to reach home. The PR aspect of the campaign outlined above might seem minor – but I'm giving praise for a softly, softly approach. By feeling that I'd somehow stumbled onto a good thing by myself, I'm more inclined to help spread the word.
Social media: Clearly, there's an online element to this campaign (it's supported by this website). But with something as real world and undigital as dirty, expensive diesel, that wasn't the first place to discover the campaign. Online and social media in this case play a supporting role – but I suspect the long tail of blog and Twitter chatter will quickly overtake the 'hit' factor of major media coverage (which has been modest).
PR Studies 
As you point out it’s the perfect time for Shell to unseat BP as the responsible oil company.
You’ve also provided an excellent example of a PR campaign aimed at the ‘easy win’ consumers. You are by your own admission ‘issue-involved’ in the sense that you’ve already got environmental and money-saving interests to cue your interest.
Where a campaign like this will struggle is with the global warming sceptic whose unlimited fuel is paid for by his boss.
The success of the campaign may depend on what proportion of the driving public have a similar profile to you.