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TLDR:
- 30k (miles) - Do not recommend Valvoline 75w90, especially with LS additive containing fluid for transmission or differential (don't need it).
- 60k - Amsoil 75w90 - good stuff, but not satisfied
- 90k - Synchromesh, pretty good stuff, even in warm weather.
- 120k - Willing to try Redline GL4 75w85 stuff
Just to chime in, I've had a variety of fluids testing for my FR-S transmission and differential. I'm nearing my 120k (mile) mark transmission fluid change as I write about it before I try Redline.
NOTE: My ambient temperatures range from 60 to 100°F, so it will not necessarily apply to most of you folks freezing your nuts off. Most of my reactions are from transmission issues; no gripes from differential, but fluid change also improved its operation a bit (quieter).
Valvoline 75w90 LS and (briefly) Synthetic 75w90:
One dealer had swapped fluids around 30k, but I wasn't happy with their choice since although being better than the initial Gunma Japan fill. Not completely happy since it didn't crunch 2nd gear as much, but 3rd and 4th were occasionally picky.
Nearing 60k mark, I asked dealer what fluid they would use for next change, and told me same one as already in use. It ended up being an 75w90 LS (limited slip additive) product INSIDE TRANSMISSION. So, not only did I had that fluid dropped, I also dropped dealer. After going to another dealer and having the TOB replaced under warranty, they swapped to a Valvoline 75w90 synthetic. Not happy still (it still crunched 2nd gear as well as 3rd and 4th occasionally), I swapped to Amsoil 75w90 GL4 on my own after the 60k mark.
Amsoil 75w90 GL4 / Amsoil Severe Gear 75w90:
I was much happier the way the FR-s transmission was feeling, but it still didn't feel perfect. Kept it for 30k miles since it still felt significantly better than previous fluids.
Pennzoil Synchromesh / Amsoil Severe Gear 75w90:
I've been using some Pennzoil Synchromesh fill on my transmission, after using Amsoil MTF for a while (still using Amsoil Severe Gear 75W90 on differential). Its the best feeling I've received from my car, since it goes into any gear with relative ease, including going into first gear without a dead stop. It is much better going into gear than with previous fluids, especially in cold weather. Alas, it is a much thinner fluid than needed. I did so because after all, my car was already out of warranty.
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