Pretty much every I2 owner on here has experienced increase in MPG as their car has run through its normal break-in process. The viscosity difference would likely be minimal if it weren't in the middle of winter so you might be getting benefits of using a synthetic as engines traditionally haven't been broken in on synthetic oils due to rings not wearing in right causing initial new engine consumpsion to never cease. Rings are typically fully broken in within less than 50 miles and likely complete by the time its out of the factory but there are other components in the engine and transmission that likely wear-in to a better fit, transmission probably more than anything.
Could it be the Amsoil, probably but the odds are that it won't be any better than another synthetic considering that the Amsoil ASM 0w20 has a viscosity on the thick side compared to other synthetics at the same full operating temperature, which is why I'm going to use it last in my lineup of oils to try in my 1st Gen even though the difference is so slight it will likely not be noticeable in the MPG numbers.
2000 MT Insight "Silver Sipper"
2000 MT Insight Silver "Clone"