I change my oil between the dealer service, mainly because I don't believe any oil can be efficient for a 12 months and a whole lotta miles.
I used to own a 1.3 fiesta and although I changed the oil every 6 months, I didn't do the filter (couldn't get to it easily enough)
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Escort 1.6GLE, Escort 1.3, Fiesta 1.3, Fiesta ST!