The truth about carbon deposits in modern engines. Details next.  

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With direct injection, a fuel injector sits inside the combustion chamber. It injects fuel directly into the chamber, at exactly the right time, with the valves closed, hence the name. Injection directly into the chamber.   Fuel is thus completely separate from the inlet air system.   In the older, port-style fuel injection system, a far less hi-tech injector was located in the inlet port, right on top of the inlet valve. (This tended to wash the valve - by spraying the valve more or less continuously with fuel, which is a powerful solvent. But it was a fairly wasteful and imprecise way of delivering fuel.)  So, with direct injection, only air goes into the inlet system. Crankcase vapours are also sucked in (via the PCV system - for ‘positive crankcase ventilation’), and some exhaust gas is recirculated (that’s called ‘EGR’ - exhaust gas recirculation).  Although they are coarsely filtered, crankcase vapours tend to be a bit oily, and recirculated exhaust gas can be a bit hot (even though it’s water-cooled). So, if you mix the oily vapours and the hot EGR, you can burn the oily vapours to carbon and give the inlet manifold and ports the equivalent of atherosclerosis.  This happens a lot if the EGR or PCV systems are defective or badly designed - so premature profound carbonisation is often symptomatic of a problem with PCV or EGR.  If you don’t drive on the open road much (for about an hour a fortnight, ballpark) your engine oil gets pretty contaminated. This is due to the hi-tech miracle of blow-by.  The PCV vapours get a bit filthier than usual when the oil is contaminated, and this can lead to the formation of significant oily, sooty deposits in the inlet tract, which is a good idea to clean up before it gets serious and impacts engine performance. It’s a problem mainly for engines that do only short trips and lots of cold starts.  The EGR tends to bake it on - but, frankly, this tends to be more profound on a diesel, which does a lot more volume of EGR as a proportion of total operational flow.  Going for a long drive every few months is insufficient to purify the oil. The open-road driving has to be reasonably regular.  To be clear - the highway driving doesn’t clean up any contamination already in the inlet plumbing. Once it’s there, it’s there. If it’s there and serious enough, you need to get it cleaned away mechanically - if the degree of contamination is likely to affect engine operation.   The purpose of the highway driving is: it purifies the engine oil and thus prevents the deposits from forming. That highway-style lean burning and sustained full operating temperature evaporates off all the water and the volatile components of unburned fuel, which exit via the PCV.   If you’re not going to do regular highway driving, get your oil changed more often - say twice a year - but still try to get out on the highway as often as you can. 30-60 minutes routinely is a good target.