2001 Toyota Highlander Burning too much oil

I have changed my oil(synthetic) every 3-5000 miles
since I bought the Highlander in January 02.
This summer it started blowing white smoke from the tailpipe, occasionally, usually after it sat for several hours. Once on the road the exhaust wasn't noticeable.
There isn't any leakage under the car and the black soot on the tailpipe is dry.
I now seem to be going through a quart of oil every 100-150 miles.
Are there any explanations apart from faulty rings, which seem unusual in a well maintained Toyota with such low mileage.

Thank you for your feedback