I chose Mobil 1 75/90 IN MY 2000 Forester. There was no measurable difference in fuel mileage after switching to synthetic gear oil. I ran the synthetic gear oil in my Forester for over 100,000 miles and changed it out at ~ 30,000 mile intervals. You will do just fine with the 80-90 gear oil. I would be interested in seeing a copy of a Subaru statement that states we should not use synthetic gear oil in our transmissions.
CX5 Touring 2.5L :-)