My 2003 Harley-Davidson Road King burns about three-quarters to one quart of oil between every 5,000-mile service.

Is this common or should I take corrective action? Modifications to the motor include a K&N air filter, a Dynojet Power Commander III and Hooker tunable exhaust slip-ons.

I hope you're not stressing out about your Road King's oil usage; one quart or less every 5000 miles is an exceptionally low rate of consumption. Most Harley owners would be ecstatic if their Big Twin or Sportster only used a quart in half that mileage.

An air-cooled engine tends to generate higher peak operating temperatures than does most of its liquid-cooled counterparts. An air-cooled big-bore V-Twin in particular can get very hot, especially the rear cylinder; which is not exposed to as much cooling air as the front. Those high operating temps also result in elevated oil temperatures that cause the oil to thin out quite a bit. With its reduced viscosity, hot oil has an easier time sneaking past the rings and valve seals, into the combustion chambers, where it gets burned and exits with the exhaust gases.

In most of these engines, the amount of oil burned in any combustion cycle is not sufficient to create visible smoke; but over several thousand miles, that consumption can add up to a quart or so of lost oil.