I have always believed that you should only put 5w30 semi or fully synthetic oil in a zetec engine. Any thicker and it does not lubricate the engine properly.

However many people argue that a slightly thicker oil works fine and is more suitable in a higher mileage engine where the thinner 5w30 will leak past the piston seals and be burnt away. Haynes lists 5W/30, 5W/40, 10W/30 or 10W/40 as all being suitable grades but warns not to go any thicker as this may cause problems. I have a very high mileage (175k) focus that has been ran for a good few years by the previous owner on 10w40. It currently loses no/ very little oil.

I am unsure whether to put 5w30 in there at oil change or stick with a 10w40. I am leaning towards a 5w30 as thats what I think should be in there, but just wondered what everyone else thought.