I get the impression that whether or not an oil company decides to market fuel saving properties of their products (be they fuel or lubricants) may depend a lot on the trade laws of the country...
Latest: from New Zealand
Shell is defending 22 charges alleging misleading advertising or engaging in conduct liable to mislead. The advertisements ran on radio, television, newspapers, a billboard and a brochure in 2006, and on a billboard and a poster in 2007.
The issues aren't that the additive didn't help, but about the size of the gain, and the comparison to a base fuel that apparently 95% of consumers didn't buy anyway.
The Commerce Commission says Shell's own testing gave an average fuel efficiency benefit of less than 1 per cent compared with a "base" fuel that few people buy
Full story: Shell in the dock over fuel additive claims | Stuff.co.nz
Related (except from Malaysia) : Shell claims new "FuelSave" gasoline formulation increases fuel economy