Transaxle fluid change

My specified transaxle fluid is 75W-85, and in doing a recent replacement, I could not find any and was told that Mobil 1 75W-90 would actually be an upgrade. I performed the drain and refill, and all went well, though now when I’m going from 1st to 2nd gear I get a little ginding type feedback from the gears. It only happens at higher RPM shifts and only from 1st to 2nd, not 2nd to 3rd or higher, nor when I am downshifting. All the other gear shifts go smoothly, like always, except the shift into second while accelerating. Can this be caused by the wrong fluid? Can this be caused by overfilling or underfilling?

People do not decide to change transaxle fluid willy nilly, I think you are experiencing problems associated with your original concern not a change in viscosity.

The transaxle fluid is scheduled to be replaced every 40K miles, so I just decided to replace it. I’m not sure if that qualifies as ‘willy nilly’ or not… What original concern may you be referring to?

It could be the Mobil 1 is too slippery for the synchronizers.
I had a similar problem with my '88 Accord.
The trans calls for 10W-40, but recent formulations of 10W-40 have different friction modifiers than they had in the '80s.
So Honda sells their own manual trans fluid.
Expensive, but made the shift-feel smoother

Does it call for GL-4 or GL-5??

On some vehicles if it calls for GL-4 then you can NOT upgrade to GL-5. Many manufacturers and sellers will tell you that GL-5 is an upgrade to GL-4. It’s not. It’s a completely different formula. It may be compatible…it may not be. GL-5 has sulfur additives which are NOT good on brass. So if your tranny has any brass in it then you’ll have to stick with GL-4.

The usual course if events is I was having this problem, so I tried this and it didn’t help, got worse etc. It seems a small minority that actually read the manual and follow maintenance advice as you have, an unexpected course of events. Congratulations for your diligence.