Given that I do not drive the mini much more than 3000k a year I tend to look at the oil and decide when to change it rather than a set km or time.
That being said does oil deteriorate over time rather than with mileage?
And having done a search on here I notice there is an old wives tale about the original mileage for oil change being 1500m. All the literature I have seen says 6000m so where did the 1500m figure come from? 

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Cheers Dave.

From my reading on the subject, yearly for low mileage cars seems to be the consensus.

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Mk 1 Cooper S 1965
If you could erase all your mistakes of the past
You would also erase the wisdom of your present
Remember the lesson not the disappointment.

Apparently the oil does deteriorate and can go acidic over time. How much it really matters is the argument, this recently came up in a Facebook group I am in (not for minis). It was basically a mix of opinions.

I would probably change it yearly for something I care about.

There is stuff I use which hadn't had an oil change for at least 20 years (TEA20). No sign of issues. It did finally get changed.

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1986 Mayfair (998 auto), 1974 S, 1974 Van, 1972 PMG Van

Too many minis

68+86auto I'm with you on this. i have a similar story with my 1960's Nuffield diesel tractor. It is possibly a story put out by oil suppliers in order to sell more oil.