First measure your mpg accurately tank brim to tank brim against the odometer. If this still works out poor, reset ECU. Changing the air filter can make a significant difference if it is well clogged, but the oil etc is not going to make a huge difference. The IS 200 was designed over 11 years ago, and the ECU etc gave the best compromise of performance, driveabilty, emissions and longevity that could be achieved with the technology of the times - which looks a bit poor now. Also remember that 6-cylinder engines are inherently silky smooth but have higher friction from the two extra bores than a 4-cylinder engine so will always be less economical. From my experience over an average 20 mpg should be easily achievable and over 30 mpg is virtually impossible.
thank you