Looking for manual transmission fluid for 2008 Mazda3i 2.0. Dealer selling for $12.50 a bottle. which one should i use the 75:90 or 85:90?

Like motor oil, multi weights are for operating temperature ranges. If living in the South (Arizona) use the higher viscosity numbers. If using the car year round in Colorado, use the lower base number. In the Miata tha we use year round I change the engine oil to lower visc. numbers in the winter and thicken the brew up a bit in the summer. My trans and diff oils stay the same grade year round.