I have the same engine in a 2004 Zetec, only mine now has 141,000 miles on it! The MPG is reasonable (40MPG)on a run if you stay below 70mph. 80mph+ or only town driving will see you struggling to get much above 26/27MPG. Change the oil every 6K and you will find the engine will go on forever. If you are getting sludging in the cap through short journeys, just change the oil more frequently. I do not know if the cap will fit but cannot see any reason for doing so really TBH. If you really want to , remove the filler tube and try it, but go carefully!
2004 C-Max Zetec 1.8 Duratec HE Petrol, 151,000 miles still going strong..