Notation for castling; Oh-Oh or Zero-Zero?

Yep, it's the letter O.
From Section 8.2.3.3 of the PGN Specification & Implementation Guide:
"SAN kingside castling is indicated by the sequence "O-O"; queenside castling is indicated by the sequence "O-O-O". Note that the upper case letter "O" is used, not the digit zero. The use of a zero character is not only incompatible with traditional text practices, but it can also confuse parsing algorithms which also have to understand about move numbers and game termination markers. Also note that the use of the letter "O" is consistent with the practice of having all chess move symbols start with a letter; also, it follows the convention that all non-pwn move symbols start with an upper case letter."
http://www.saremba.de/chessgml/standards/pgn/pgn-complete.htm#c8

Thank you EscherehcsE
In researching the Evans Gambit on Chessbase, I found the downloaded version of this match (via https://database.chessbase.com/ ) :
[Event "Titled Tuesday intern op 25th May"]
[White "Zhigalko, Sergei"]
[Black "Costachi, Mihnea"]
[Site "Chess.com INT"]
[Round "10"]
[Annotator ""]
[Result "1-0"]
[Date "2021.05.25"]
[WhiteElo "2572"]
[BlackElo "2471"]
[PlyCount "35"]
7. 0-0 Nge7 { Notice castling uses zero-zero not the standard letter notation O-O }
This "error" may or may not be flagged by certain PGN readers. This discrepancy has appeared in 5 different files I have examined. PGN Mentor has an auto-correction filter in its reader, but other programs may not. Until standardization took place, I suspect many transcriptions used zero-zero unintentionally.
Is the notation for king-side castling Oh-Oh or Zero-Zero ?
I realise that this does not matter in handwritten notes, but which should I use if I am typing up my notes?