https://www.wikitree.com/wiki/Help:Sources#Second-hand_information
Second-hand information
Schmoe, Joe. Personal recollection, 3 Jun 2011, as told to [[Example-6|John Example]] via the phone. Notes in the possession of John Example.
Often genealogical information is provided by other family members.
It's a good idea to include who said certain information, and if you remember, when and where they said it. You may also want to include a link to the person's WikiTree profile page.
First-hand information (yourself)
[[Example-6|Example, John]]. Personal recollection, 5 Jul 2015.
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In your case, I would write it out as it sounds in your text above:
Interview of Great Grandfather xyz in March 2018. Notes in possession of your name.
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You can find an example of some I've done here:
Elmer Herbert Ranck - sources 9 & 16.
9. ↑ Ranck, Paul E. Personal recollection, April 1993, as told to SJ Baty in an interview in Callahan, California. Notes and video recording in the possession of SJ Baty.
16. ↑ Ranck, A. Personal recollection, October 1997, as told to SJ Baty in an interview in with A Ranck. Notes in the possession of SJ Baty.