synthetic oil will help marginally with MPGs but most benefits are engine longevity. Oil breaks down with moisture (condensation), gas blowby and heat cycling....synthetic has been proven time and again to outperform any conventional oil because of its synthetic additives. Do it for peace of mind and longer change intervals. I swap 5K or 5Months....usually 5K comes first and the oil hardly ever looks black. Driving habits will change the requirements if you are a lead foot, excessive idler, and short trips without giving sufficient time for the engine to maintain operating temps.