Ford recommend no need to renew diff oil, unless you have been towing or the car has been worked hard in a hot environment.
I just changed mine, easy as. Best to give it a good run and drain it when the oil is hot. I used a suction gun to refill it, didn't even jack it up. Will hold about 1.6 litres, use the oil recommended in the owners manual.
I used Castrol SAF-XA full synthetic stuff, and at $26 a litre, this will be the first and last time!!
If you bought the car 2nd hand, it is probably worth changing it at least once.
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