You always check the oil with the engine warm but you need to be on level ground and leave the car for 5 to 10 mins to let all the oil drain into the sump.
As Chris said, oil expands when it's warm so if you check it when it's cold and top it up to the max mark it will expand above the max mark when warm and start burning oil.
Make sure you use the right oil when you top up as well, do not mix oils in the sump.
Best to go to Ford and get a litre of the correct oil to top up with.
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2011 Focus 3-door Zetec 1.6 TDCi.
Previous <img src="/images/graemlins/stemo.gif" alt="" />s - 2006 Panther Black & 2007 Red. Gone and quite forgotten <img src="/images/graemlins/lol.gif" alt="" />