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Can red oxide under acidic conditions become a black oxide?

Q. I tested an oil sample that had shown red oxide as extreme, 9 on a scale of 0 - 9. It also had dark metallo-oxide as 0. In addition, this oil had an acid number of 1.8 per ASTM D974.
I took an additional sample of the same batch and sent it to a different lab for testing 4 weeks after the initial sample had been sent in. The new results were 5 out of 10 for black oxide, and red oxide 0. The AN went down to .374.
Is it possible that the small amount of acid in the oil turned the red oxide to black oxide, and became almost neutralized?

Richard Greenwald
Lubricant Sales - Beachwood, Ohio USA
March 22, 2018

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