In general, an Infinity should be serviced every six months or 5,000 miles, depending on what comes first. For a major servicing, which goes beyond an oil change, the recommendation is 10,000 miles. However, the owner's manual has more detailed information about the frequency.
Servicing includes changing the oil and checking other fluids, replacing the fuel filter and spark plugs, and checking the ignition timing and sensors. Getting the tires rotated is also recommended during an oil change or tune-up. Most Infinity dealerships recommend using a synthetic oil, as it is resist to breakdown due to high temperatures and is more stable in general.