Replacing oil seals on vehicles is inevitable to prevent leaks
In the distant past oil seals were made of leather, clamped in a steel casing.
Some present-day seal makers still have names reflecting this history, such as Chicago Rawhide, one of the biggest.
On modern cars the oil seals are made of advanced synthetic materials and incorporate design features that make them efficient and long-lasting.
But nothing lasts for ever. As a car gets older the flexible sealing elements lose their elasticity and the sealing edges gradually wear away. Sooner or later you spot an oil leak and you know it's time to replace a seal.
An oil seal can often be replaced by the home mechanic relatively easily.
There are exceptions, such as certain seals on front-wheel drive cars, where major dismantling work is required to gain access to them. But on a rear-wheel drive car or bakkie, the oil seals that most commonly need replacement, the front crankshaft, gearbox output and diff pinion seals and the front wheel bearing seals are easily reached.
The rear main oil seal on the crankshaft is a different story. To replace it, you have to remove the flywheel, so we normally try to live with a leak there until the engine or gearbox has to come out for other reasons.