If you have been changing the oil regularly there is probably no appreciable wear on the engine, it's old but not worn at 5000 miles per litre.

Slightly off-topic, but am I alone in feeling that 1000 miles per litre of oil is extraordinarily high to be 'acceptable' - or have I just been lucky? My ancient Citroen 2.2 diesel (15 years and still going nicely) has never used any noticeable amount of oil between changes at the shorter of 1 year/10,000 mile intervals, always using the correct synthetic oil. I do check the dipstick periodically because things might change but it has been reassuringly predictable so far. What is the experience of others?

To put off throwing stuff out, which I hate, I'll have crack at the "thin v. thick" chestnut.

This is an oldie but mouldy, and, like many basic automotive questions, hard information on it is rare, but as a generalisation the above "Thin gooood, Thick baaaad" concensus is simplistic and mostly wrong.

Firstly, the "American Culture" thang. This picture of American motorists as good ol' boys keen on duelling banjo's, chawin terbacca, 1957 Chevvies and thick mineral oil is attractive, but largely bogus. American motorists will probably generally follow the owners manual almost, but perhaps not quite, as uncritically as British motorists, and owners manuals designed for the American market generally insist on skinny oils, skinnier than the same car would specify for the Australian market, for example, where 20W50 is still commonly recommended and used.

This difference is probably largely due to the US government CAFE regulations, which impose "fleet" fuel economy targets (with associated penalties) on manufacturers.

Skinny oils give marginally better fuel econony due to the lower initial viscosity. I suspect wide range multigrades (say an 0W40 compared to a 15W40) may also give marginally better fuel economy at the final viscosity due to the temporary viscosity loss behaviour of viscosity improvers, but I can't remember if I've ever seen confirmation of that.

The tradeoff for this fuel economy benefit, is a lower level of protection. Lower viscosity oils give a lower minimum oil film thickness, and in locations where contact can routinely occur, like cam lobes and piston rings, they will spend less time in hydrodynamic lubrication, where it doesn't, and more time in mixed and boundary layer lubrication, where it does, amd where wear is limited by anti-wear agents and friction modifiers which coat the surfaces.

This does not mean that skinny oils give inadequate protection. Most cars are scrapped long before engine wear becomes an issue, and the tradeoff is probably entirely rational for the vast majority of new car purchasers. Nevertheless, the protection is better, and the oil consumption lower, for thicker oils. Since these generally use less VI's they also tend to be more resistant to shearing and less likely to gum up piston rings.

The usual counter argument is that thinner oils give better protection because they get to the top-end quicker and/or circulate faster. Most if not all oil pumps are "positive displacement" which means that, provided the pumpability limits of the oil is not exceeded in extreme low temperatures, they will deliver a fixed volume of oil per revolution, irrespective of oil viscosity. In practice there is some "leak back" of oil past the pump gears. This leakage increases with pump wear and decreases with oil viscosity, so a worn oil pump will deliver less skinny oil and generate a lower oil pressure with it.

For a given oil pressure, oil flow through an oilway will be greater with skinny oil, but this is unimportant, since flow doesn't lubricate. Flow removes heat and debris but these are not usually limiting factors.

Note that I'm generalising here. I know nothing specific to the Ford Zetec and am not saying you should put thicker oil in it.