Too-high oil viscosity

So, the engine has warmed up to operating temperature, but the oil's viscosity hasn't dropped to the required (design) figure — what happens? Under normal revs and loads, nothing much, really — the engine's temperature rises a bit and the viscosity drops to the required norm, which the cooling system will then handle. In this case, the engine's operating temperature will be higher than the norm for that particular rev range and load, but it will most likely still fall within the allowable range. The other issue is that the engine will spend more of its time running at a higher temperature, which certainly doesn't help extend its service life.

It's a completely different matter if you, say, sharply increase the engine's revs (an emergency burst of acceleration while overtaking on a long uphill stretch, for instance). The shear rate jumps sharply, and the viscosity doesn't match the current temperature (again, this comes down to the engine designer's calculations), so at that moment the engine has to warm up a bit more (to a higher temperature) to bring the oil's viscosity down to an acceptable level. And at that moment, the temperature of the oil and the engine could well cross the maximum safe limit.

The upshot of all this, put in terms an ordinary car owner can follow, is roughly this: if the oil's viscosity is higher than the norm set by the manufacturer, the engine constantly runs at elevated temperatures, which wears its parts out faster. What's more, operating temperature also directly affects the service life of the engine oil itself: the higher the temperature, the faster the oil oxidises and becomes unusable. So an oil like that also needs to be changed far more often.

Either way, you won't be able to notice or feel any of the negative consequences of overly high viscosity without complex measurements and opening up the engine, over a relatively short period of time — it won't show up after 10 or 20 thousand km, but rather after 100 to 150 thousand. And proving that the cause of increased engine wear is specifically an unsuitable engine oil is practically impossible — which is why many mechanics, and even official service stations, often don't pay much attention to whether the viscosity of the oil they're pouring in actually matches the carmaker's requirements for that particular engine. Remember — it's in their interest if your engine breaks down once the warranty period ends, even if you don't come back to them for the repair!