There's your problem. The ECU doesn't know what octane you're running, it just knows if it's detting or not. It takes several tanks of 98, or an ECU reset to get a difference in gas milage.
If economy is an issue, yeah, give it a big service. New air filter, upper engine clean, new fuel filter, new spark plugs, Oil and filter change (Do your oil and filter every 10,000K's at least) Maintenance is key.
no way its has to be 5,000k's i have lots of mates do oil at 10,000k's and there do beb i do oil 5,000k's and never done a beb