Engine oil was once changed several times a year, and many modern cars ask for it far less often. The change reflects genuine improvements in three separate areas rather than a relaxation of standards.
What the oil is actually doing
Oil separates moving metal surfaces with a film thin enough to be measured in microns, and that film is what prevents wear at the bearings.
It also carries heat away from hot components, holds combustion by-products in suspension so they can be filtered, and neutralises the acids that combustion produces.
Oil is replaced not because it stops being slippery but because the additives handling those secondary jobs are consumed.
Why synthetic base oils changed the maths
Mineral oil is refined from crude and contains molecules of many different shapes and sizes, some of which break down or evaporate more readily than others.
Synthetic base stocks are built to a consistent molecular structure, so they resist thermal breakdown, hold their viscosity across a wider temperature range and evaporate less.
A more uniform base also gives the additive package a more stable environment, which extends how long those additives remain effective.
How engines themselves became cleaner
Tighter machining tolerances and better piston ring sealing reduce the amount of combustion gas forced past the pistons into the sump.
Less of that contamination means less acid and less soot entering the oil, so the additives that neutralise them last considerably longer.
Improved filtration and better crankcase ventilation remove more of what does get through, compounding the effect.
What condition-based monitoring adds
Many cars no longer specify a fixed interval at all. Instead they estimate oil condition from engine temperature, load, running time, cold starts and distance covered.
A car used for short cold journeys will reach its service point far sooner than one covering long steady distances, which reflects the real difference in how the oil is stressed.
This is more accurate than a fixed figure, which necessarily had to be conservative enough for the hardest use anyone might give the car.
Where long intervals still cause trouble
Turbocharged engines run bearings at high temperature, and oil that has degraded can form deposits in the turbocharger's oil feed, which is a known failure route.
Engines with variable valve timing and similar oil-actuated mechanisms depend on clean oil passing through fine passages that block readily.
Owners doing short, cold journeys are the most exposed, because their driving pattern is exactly the one that dilutes and contaminates oil fastest while producing little mileage.