You likely won't see oil fouling until consumption gets down to a few hundred miles per quart. When changing valve seals a competent mechanic can tell if the guides are excessively worn and would suggest replacement to the owner.

So what is your measured rate of oil consumption?

If it's not oil, the other culprit could be gasoline fouling due to misfiring.

Check the plug wire! Improper removal of the wires (without using a special pair of "pliers" to remove them by firmly grasping the metal RFI shield) can damage the internal connection between the RFI resistor, wire, and terminal.

Duke