Idemitsu Racing Gear oil 75W-90
 

Anyone know if this gear oil is okay for our transmissions? I have searched and found nothing. I believe GL-3 is recommended for the trans as far as I recall. Pretty sure it's fine for the dif. Going to the track on Sunday and really need to change the fluid and it's what I have on hand. Please let me know what you think. I couldn't even find if it's compatible with gold metals. Thanks for any knowledge you can share!

I wouldn't use it for the transmission considering the back says only GL-5 (google search). I would consider using Motul 300 or Redline MTL or MT90. For you, I'd consider the MT90 over the MTL. Since you said Sunday is your track day, I don't think you have many options at advanced auto, etc. But you can give it a look!

I changed the OE fluid to motul 300 (trans and diff) at 1k miles I'm at 30k now. Should I even worry about changing it for the track day? The idemitsu should be okay for the diff right?

I think choosing motul 300 for the track day is perfect. Motul 300 performs well once it warms up and a track day would be perfect for that. The maintenance interval for transmission fluid is like 30k, I mean I think it will be fine. Maybe change it after the track day. In regards to the diff, I feel like it will be okay. Fact is, if you go to Subaru Parts, they use 'Subaru high performance gear oil 75w90'. They would put this in to service your transmission and diff if you wanted to. Its a GL-4/5 rating and its made by Idemitsu.

Well, I guess I could be a guinea pig... Lol. On another note, this fluid is recommended for Miata, Rx7, and Rx8 transmissions. Those are also Aisin, so I'd have to imagine it would be fine. Wouldn't be the end of the world if I had to pull the trans and put in new synchros lol. I know that's not a happy time for most people, but I'd be okay with it, not stoked, but okay. Going to have to do it for my 96 7.3 powerstroke's ZF-5 soon lol. Hopefully I can make it another 30-40k on that but they are likely original. Any idea how long it would take for the synchros to be eaten by a GL-5 fluid if it's not compatible? Anyone had it happen?

It would just be easier to get a new transmission when it comes time to replace your syncros. A new transmission is like $500.

So, I ran it in the trans and diff. I actually like the fluid for the diff. I feel like it made the diff more responsive. The trans feels about the same, except first gear is a little tricky to engage while moving. I am still waiting to hear back from the idemitsu rep about the yellow metal compatibility. Might switch back to the motul 300 for the trans, but I'm keeping the idemitsu in the diff.

After dailying the car a bit I take it back, the fluid is great. Feels better than the 300 once it's warm. Behaves about the same as the 300 when cold except for the rolling first gear needs a rev match. Maybe 300 was the same and I'm just paying more attention now. Rev matches are buttery smooth everywhere. Idemitsu also responded, it's fine for yellow metals. If you're changing your trans fluid soon and want to try something different I'd use it again. It's also about half the price of 300, so that's nice.

Did you specify for our make and model, or just "yellow metals"?

I eyed this oil a long time ago, but opted not to go for it because of the API GL was missing on most sites (except the one you linked was GL-5 which shouldn't be used in our trans).