Engine oil running low, lost about a quart of oil driving 500 miles. on 2001 Lexus RX300

No visible oil leaks, but black carbon soot on exhaust pipe

Asked by for the 2001 Lexus RX300
You probably have a front or Rear enging seal leaking. Check where you Park and see if there is oil residue. If no leakage detected you may have a Valve Seal defective. Have someone check your Spark Plugs to see which one may be fouling out because of oil seapage. This will narrow the search on the leaky valve seal.
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See if your engine has a PCV valve and if it does, replace it.