Project Farm on youtube did a fun synthetic oil showdown. The differences between a "good" vs a "bad" synthetic oil were pretty surprising. I switched over all my small engines to one of the better synthetics and on a cool or cold the day the difference in effort yanking over a pull start engine is night and day. That has to translate into cold weather mpg (not to mention engine wear) at least during warm-up. I think once the oil is up to temperature the difference is much more minimal.