Replaced transmission fluid, now it's causing problem
 

Hello,

I recently replaced both manual transmission fluid and rear differential with Amsoil products. Prior to changing, I rarely had grinding but it's now grinding when I downshift from 6th to 5th at speed above 65mph. No noise at speed below 55mph.

Mileage: 13000
Trans fluid: Amsoil Manual

Rear Diff fluid


The funny thing is that it started to grind right after changing the fluid. I am kind of stumped right now why it's happening. I've searched but most thread shows the common problem in 2nd, 5th gear grinding, but that's with the factory fluid. Should I try some other lubricant? I've also read that some people here with BRZ use GM Synchromesh manual transmission fluid or Pennzoil. I don't find a possible way to mess up a transmission while doing a flush. It's simple as drain and refill. Any suggestion/help is appreciated.

Sorry I don't have a solution, but I can at least give you the satisfaction of assuring I just read this entire post in Kevin Malone's voice. That should count for something.

I miss the office. I'm going to watch some funny moments.

I think you'll find it's a coincidence. Mine was the same and the gearbox is being replaced this week.
The factory oil isn't special and you used good to oil to specification.

You changed the oil to hopefully fix the grinding you were experiencing and it didn't work. Take it in under warranty and you should get a new modified[hopefully] gearbox too.

one more note, I've overfilled the fluid by .3 liter.. not sure if that could cause too much lubrication, resulting in "slippery" when downshifting?

How did you manage to get 2.5L in it through the filler/level hole?
I doubt it would cause any problems, especially synchro problems but it's easy enough to take the level plug out and drain off the excess[on a hoist].

the car wasn't 100% leveled. Front was a bit higher than rear, so I guess the trans box took more fluid in the back.

Here is an update. Replaced fluid with Redline MTL and the noise is gone.

Since the grinding sound came RIGHT AFTER using Amsoil, it had to be the fluid.. So I decided to wait for about a week thinking that the fluid had to be "broken in". Well... still same grumpy downshift from 6th to 5th. I did some research about oil weight/viscosity/lubrication stuff and concluded that Amsoil is providing too much lubrication, which could lead to too much slippery in synchronizer. I may be wrong on some points./ but tried Redline MTL. I've read from a thread here that it provides a grip to synchronizer. I guess it works!

Amsoil was great, providing smooth shifting, but it just didn't work on downshift and I don't know why it only happened on 6th to 5th downshift.

Well that's interesting. Strange though, that it should only affect 5th gear?

It makes my new gearbox for the same problem look like overkill!

I have no idea if it's affecting only 5th gear. I think I'm the only one who is experiencing this problem after replacing trans fluid with Amsoil GL-4 (not synchromesh). I also noticed grind when I upshifted from 4th to 5th gear above 4000 rpm. I've read numerous threads where people had grinding on 5th gear, but those people are with factory fluid. So the overall problem with this gearbox seems to be 5th gear. I'm wondering if anyone who still has problem with 5th gear and with stock fluid can try using Redline MTL (75w-80) and see if the problem disappears. It's only around $50 for 3 bottles, but not sure if anyone is willing to try before taking the car to dealer.

You said in your case it was only affecting 5th gear which was what I was referring to.

The problem with that oil is it's not to the 75w-90 spec if you get any problems in the future. I supplied the oil to the dealer and they changed it for no cost after approving the spec.

Very interesting still that oil caused and cured the problem.

Yeap, it was only affecting 5th gear.. I'm confused with Amsoil product. I know some people here use Amsoil in their transmission, but I haven't seen any problems or complaints.

Maybe a bad batch slipped through?

It's difficult to imagine how the oil has such a marked affect on the operation of the synchros?

I doubt its a bad batch.. More like the correct formulation of friction modifiers were needed.
My old camaro had issues with the diff.. The posi traction aspect wasn't working
Most of the time.. Only one wheel would spin during hard launches.. Like a non posi diff works.. I changed out the fluid and used redlines diff fluid with their friction modified formula.. It was like new again! Both wheels hooked up and cars tail went sideways and left two nice rubber tracks.. I then became a believer in the power of fluids! I have been using Redline fluids in the trans and rear ends of all my cars... With excellent results for the last 30 years..
Their stuff is pricey, but it works and lasts a long time..