I'm writing a paper, and I quote several lists from a source that doesn't use Oxford commas. Sic seems way too strong for the situation, since there's no real error in the quotes, just a different convention. I could change my own paper to be consistent with the quotations, but then what would I do if I had to quote a source that did use Oxford commas? I could sneak the commas into the quotes. Finally, I could just leave the quotes inconsistent with the rest of the paper. What's the correct way, if any, to address this?
This is what I tell my students in similar circumstances.
If you quote, reproduce the author's text exactly. If the author's text not follow your punctuation conventions, the matter hangs over his or her head, not yours.
Unless the quoted passage contains an egregious error, do not call attention to it with "sic."
Some editors will enforce conformity on quoted text. If so, the sin is theirs.