I don't know about you guys, but I thought the Kizashi's shifter with the factory fill was rather disappointing. Other than the medium throw and somewhat imprecise shift path, I thought the gear shifts felt "watery". Changing the fluid to MT-90 made shifting significantly better and feel more buffered. Maybe it's a personal preference, but I liked the difference MT-90 made. Also, MT-90 didn't make shifting in cold weather all that much worse than the factory fill. Last winter, I was driving around in 7 F (-14 C) temperatures and my shifter did not disappoint. Maybe a different story in -30 C weather like in the Mid West and Canada, but most owners don't experience those kinds of temperatures.
Another thing is that I hear quite a bit of gear rattle when in 2nd gear and under speeds of 20 MPH. I hear it occasionally in stop and go traffic and commonly when coasting in 2nd down residential streets. I've heard it with the factory fill and even so with MT-90. Anyone else's Kizashi make this noise? I assume it's rather normal because my Mazda3 did something similar.
Lastly, I want to note that MT-90 made absolutely no measurable impact on fuel economy. I changed the fluid almost exactly 1 year after buying the car. Until that time, it had the factory fill. It's now 1 year since I changed to MT-90. My fuel economy averages have not changed by any noticeable amount.
What weight is best for you mostly depends on where you live and what you want your shifter to feel like. I'm not a fan of watery and light-feeling shifters. As I've said before, the Kizashi's shifter is a downgrade compared to the one in my Mazda3 which was notchier, heavier, and far more precise. Switching to MT-90 in the Kizashi helped the shifter feel better for me.
If you live in a place that gets very cold, maybe 75W-80 is better, but theoretically, it should not make much of a difference due to the fact that they are all rated 75W meaning they all have flow characteristics of 75 weight in W (Winter) meaning cold temps.
All this being said, I may try MTL next to see how I like it and how the car likes it. I just simply think using MT-90 in a car "recommended" to use 75W-80 isn't a big deal especially since Suzuki says the weight is acceptable.
It may be that the only benefit MT-90 brought was the more buffered, smoother shifting feel, but that's exactly what I was going for.
2011 Suzuki Kizashi Sport GTS 6MT (Black)