There is one issue which would cause the EGR code to set when there is not a problem with the EGR system; that would happen if the ignition timing was set improperly. It is necessary to disable the electronic spark advance circuit before adjusting the timing; and also to use a timing light; rather than setting it by ear.
When I recommended using Chevron Techron; I did not mean to use their fuel. Techron can be purchased separately, in a black bottle, from (of course) Chevron stations, Auto Zone, Wal Mart, and O'Reilly parts stores. Chevron fuel has also had another additive, called MTBE in it; which was determined to cause cancer, and was banned in some states. They continued to use that additive in California; and I don't know whether they ever stopped. So I have gone to Shell (which used to be the worst gas on the market) but which has since dramatically improved, now outperforms other brands, and does not contain MTBE.