Following the advice of so many of you on this thread, I replaced the PCV valve on my daughter's '99 and changed the oil to Pennzoil Platinum. On her return trip to college, instead of using more than a quart of oil every thousand miles of highway driving, we used none. Zilch. I'm so pleased. I'm not sure if the credit goes to the oil, the PCV valve, or both, but I'll keep using this new oil.

The credit really goes to all of you, without whom I wouldn't have come up with a solution. Thanks, everybody!