[SaabJohan]

Using the thinnest oil your engine can take will reduce friction and decrease fuel consumption.. You can also buy an oil which have API EC and/or ILSAC GF-2 classification, this means that the oil have shown to reduce fuel consumption compared to a reference oil.
NEVER use any addetives in your oil.

Leaning out the fuel and advance ignition timing will also decrease fuel consumption, but note that fuel injected engines run lambda 1 and running leaner will result in that the catalyst won't work as intended anymore. You can also advance engine timing, but this aswell as the leaning of the fuel means that a higher octane fuel will be required.