Most of the well conducted tests have shown all the spray cleaners, including Seafoam and CRC, don't do much with even modest carbon build up. Some dealers love to charge for these cleanings as it's yet another way for them to pad their revenue (and all the service writers work on commission) but the "before and after" pics of the back sides of intake valves rarely show any significant improvement. Some early D.I. engines, especially from VW, Audi, and Ford, have serious problems with carbon build up even in 20K or 30K miles. More to the point for this forum, using a very low volatility full synthetic oil should significantly help reduce carbon build up. The lubricant industry has a been a bit slow to respond but I think it's going to become arguably the most important criteria for choosing an oil in a direct injected engine (beyond using the right viscosity and service rating). The new GF-6 standard should help.