I think it's pretty rare to get a cat efficiency code. Mixture codes, especially with older vehicles, are often a combination of issues which is why sometimes even a small change of one of the factors can be enough to bring the mixture back within limits for turning the CEL on.

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1988 325is (purchased new) sold
2004 X5 3.0 2005 X3 2.5
2008 X5 3.0 (new to me)