My nephew is rounding 4,000 miles on his 2013 GTI. Saw it for the first time yesterday and noticed the exhaust tips have a lot of black soot... Thick and dusty. Pretty sure he's running '91 octane and probably does mostly around-town driving but, like any teenager, wouldn't surprise me if he nailed it once in a while. What causes such a heavy build up like that? Is it something that can rectified? Is it a problem? Also, the engine compartment referenced an oil spec I never heard of : 502. Might have been VW 502? Does that spec translate into something more commonly found here in the US? I see lots of turbo guys using a 40 weight. But with the hills in our area and the short--trip driving, would a 5w-30 or something be too light? I'm about to google 502 oil so I may have an answer for myself shortly but would Appreciate any input Thank you One last thing: Direct Injection. This is my first experience (not mine really but I want to help the kid) with a DI engine. are there any preventative measures he/we can do to help prevent the engines potential for carbon bulld up?