You must use 5/30 oil with the ZETEC is your in trouble. Are you using the Mondeo PAS pump? If not have you changed to a 1.8 water pump? Unless you use the modeo PAS pump, you must change the water pump on a silver top. If its a black top there is another work around that needs doing if you lose the PAS.

With the wasted spark system, if one cylinder is not firring it is more likely to be a fuel problem, knackered injector etc. Buy yourself an Bosch 2 pin Noid light from Partco and do some tests.