Manual Transmission Fluid

I recently purchased a 2010 Mazda 3 with 42,000 miles on it and it has a five speed manual transmission. I wanted to change out the fluid with a synthetic like Mobil 1 or something like that. I live in northern Wisconsin where we can see -20F during the winter. I stumbled upon a TSB from Mazda (05-009/10). It says the 2010 and newer manual trans calls for 75W-80 gear oil. It says that if you use 75W-90, it may result in reduced fuel efficiency and/or deterioration of shift feel when cold. I e-mailed a technical rep at Mobil and asked him about this. He replied that Mazda is right and I should buy the 75W-80 from them, becuase Mobil only makes 75W-90. I checked around and could not find any synthetic 75W-80. I think the Mazda stuff is regular non-synthetic. What does everybody think? I still like the idea of using a synthetic, even if it is a bit thicker.

I ended up putting in the full synthetic Motorcraft 75W-90. List price was like $23 per quart, I still paid close to $18 per quart for three quarts. I have been very pleased. It has been nasty cold this winter in central WI, down to -30 some nights. I mean sure, it does get somewhat thick at those temps, but nothing like an oil with that heavy of a weight normally would. And after just a coulple miles of driving, it gets better right away. Very nice shifting all of the time.