Again folks, time is mandated because of acid and contaminates caused by combustion. If motor oil is not changed regularly, carboxylic acids raise oil acidity. The hydrocarbons found in good motor oil contain carbon-carbon multiple bonds. Of greatest importance are the double bonds. Written –C=C–, at high temperatures these bonds react with oxygen molecules (O₂), cleaving them into shorter molecules called carboxylic acids, named for the carboxyl group, –COOH First, the molecule has been split into two smaller molecules. This results in a decrease in oil viscosity. Second, the new molecules are not simply hydrocarbons, but are acids; they erode slightly exposed metallic surfaces. Finally, the metal that was attacked forms salts with the carboxylic acids, liberating hydrogen gas. But of what harm specifically, are carboxylic metal salts? They couple with other substances resulting from the breakdown of oil additives, fuel traces and other substances, to produce ‘coffee-grounds’ sludge, complex tars, varnishes and resins. These substances can coat piston parts, causing them to function less efficiently. Also, these oxidation products are largely insoluble and tend to settle out, hindering motor oil flow. This of particular importance in engines that use very ‘low-weight’ motor oil that must reach hard-to-access locations.

Easy answer is, change the oil regularly.

 

Good point Box.
Seems to me however that this acidification results mainly and progressively from exposure at high temps (so during engine running) and adds up along these runs.
So would that be less detrimental in a car doing only 1000 km a year during say, 20 times a 50 km trip?
Of course oil residing still in the engine is not favourable either to avoid sludge...

 

The weak link is the oil filter. Some are junk while some have premium internals. Some are designed specifically for an extended 10K interval with no mention of time duration.
To be on the safe side, change every 12mo/10k mi.
It's cheap insurance and peace of mind.

Don't forget other filters/fluids: tranny, fuel, power steering, differential, supercharger, air-cabin filter & anti freeze,

 

The reason why you change the oil every year, if the engine is not run or it's not driven, is to drain off the water that will condense and collect in the pan.