If the manufacturer says use 20W40 synthetic, use 20W40 synthetic. It costs you all of 20 bucks more for your once a season oil change and trying to save that doesn't make a lot of sense.
Yes, there's a difference between 10W and 20W. Basically those numbers mean a 10W40 oil flows like a 10 weight and protects like a 40 weight, not that I'm a chemist to understand what's really going on. Running a lighter than recommended viscosity can result in lower oil pressure, particularly at idle once hot.
Wrong about the carbon, conventional oil doesn't contain it and leave deposits. It's just a different chemistry. Listen, you could run conventional 10W40 for 20 years and not have a problem. But why chance it? Run what the manufacturer recommends and you are taking the fewest chances on the life of your engine.
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