Every tailpipe goes black. The useful question is what the black is made of and how it got there. Dry carbon and an oily film look similar in a photograph and behave completely differently under a finger, and that one difference decides whether the pipe is telling you about combustion or about wear.
Judge it cold and judge it after a wipe. Wipe a clean cloth around the inside of the tip, look at what comes off, and leave the car standing long enough for anything volatile in the deposit to evaporate. A reading taken hot is a reading of the last few minutes only.
Dry carbon is matt, light, and comes away as a black dust that leaves grey on the cloth. It does not cling to the skin, and it brushes out of the tip with a dry rag. Under the film the metal is unchanged, and the boundary between coated and clean is soft.
That is normal on any engine, and more so on one that spends its life in traffic. Short runs keep the system cool, the last section never reaches the temperature that would burn the carbon away, and the film builds slowly for years without anything being wrong.
Oil leaves a film that is glossy while wet and tacky once cool. It smears rather than dusts, it darkens the cloth in streaks, and it holds grit. Wiped clean, the metal underneath often shows a brown or blue stain where the film has baked on at the hot end.
An oily tip means liquid reached the outlet, which means it travelled the whole system without burning. The engine is the first place to look, but not the only one: a turbocharger seal, a breather routed into the intake, or a badly worn valve guide will all put the same material out of the back.
Fuel residue is thinner, dries faster, and smells unmistakably of petrol while the tip is still warm. It usually appears after a change to the calibration, a sticking injector, or a lot of idling from cold, and it often clears on its own once the car is used properly.
Oil is thicker, persists, and smells of hot lubricant rather than solvent. It tends to appear gradually and to worsen, and it is usually accompanied by a level that falls between services. The smell and the rate of loss together are more reliable than the appearance alone.
A crankcase breather vented into the intake puts a fine oil mist through the engine at all times, and some of it will always reach the exhaust. On a modified car with a free-flowing intake this is more visible, because the mist is no longer diluted by a restricted intake path.
That is not a fault and it does not call for a rebuild. It does mean that a lightly oily tip on such a car should not be read as wear until the breather route has been checked, and until the oil level has been watched over a reasonable distance rather than a single week.
Cleaning is not cosmetic here. A tip that has been wiped gives you a starting point, and the pattern that forms over the next few hundred miles is far more informative than years of accumulated film. Clean it, drive normally, then look again.
Use a solvent that evaporates fully and let the tip dry before the engine runs, or the first heat cycle will bake the cleaner into the deposit and spoil the comparison. Where the tip is a finished component rather than a cut end, it deserves the same care as any exhaust pipe joint, and matching the fittings to the other fabricated parts on the car keeps dissimilar metals out of the assembly.
