Oil level in my 2018 Hyundai doesn't drop and is always black

Modern engine oils contain lots of additives (including detergents) to protect an engine from wear as well as keeping it clean inside. In many cases, this internal cleaning process is what makes the oil go a darker colour over time.

That said, I'd be surprised if an engine with just 40,000km on board is dirty enough inside to make the oil go black. Have you owned the vehicle from new? Did perhaps, a previous owner skip oil changes that allowed black sludge to build up inside the engine?

But equally, don't confuse discoloured with black. Check the oil when the engine is still warm. Hot oil should appear a slightly lighter colour than the same oil in the same engine when it's stone cold. Wipe some of the oil on the dipstick on to a clean piece of paper towel. Does it still look black, or is there a brown-ness to it? If it truly is black than you might need to switch to a high-detergent oil and give the engine a couple of quick oil changes (say, 1000km apart) to see if that cleans things up a bit inside.

Now having said all that, the question becomes one of whether your car is a petrol or diesel-engined i30. If it's a petrol engine, the above advice stands. But if it's a diesel engine, then ignore the above and stop worrying. That's because diesel engines are inherently filthy things which eat a proportion of their own exhaust soot and are, therefore, notoriously grotty, turning their engine oil black in seconds. In fact, by the time the oil has been poured into the filler cap and drained to the sump – before the engine has even been started – it's black. This is quite normal.