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Discussion Starter #1
I recently started driving my Mustang project. I installed a TKO-600 trans behind the 351 Cleveland.

I was reading online comments and watching videos about this trans. A lot of people were saying the fluid should be changed after break-in. Yet, the tech sheet I got with the trans said most street applications will be "fill for life". I used fluid I got from Modern Driveline which was a full-synthetic from Amsoil. That stuff is not cheap. I don't want to drain it if I don't have to, but will certainly do so if necessary. What say the VMF crowd?

Also, I've noticed I can get some grinding when I shift from 5th to 4th. If I briefly pause between this downshift, no grinding. Is this typical for a TKO-600? Or, can I expect it to go away after break-in? The trans currently has just over 300 miles on it.

Anyway, I really like this transmission! Contrary to what I've heard, I don't have any trouble shifting from 2nd to 3rd. I just push up on the ball with my palm and it goes right into 3rd. And boy is it nice to shift into 5th on the highway and drop 1,000 RPMs.

Thanks.
 

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I've heard different things about the oil for the TKOs. First, allowing a traditional fluid to be used for "break in" makes sense in the sense that if any small filings are released, you can easily change that after the break in period. I've heard several debates about the GM synchromesh vs traditional transmission fluid vs Amsoil. I'd follow MDL's recommendations, or call them for advice.
As far as the grinding from 5th to 4th, again, seek professional advice (MDL) but it may be that your trans is out of line just enough to cause this problem. I was convinced I needed a rebuild (or at least new reverse synchros) because I sounded like a 15 year old learning to drive any time I started in reverse, and frequently got locked out of first gear if I stopped while in second (pulling up to a stoplight). My mechanic made a minor adjustment to the drive angle and everything was smooth as butter (and he saved me $1k).
 

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I used a Royal Purple syncromesh substitute when I first got mine. After a while I switched over to the actual GM syncromesh. Shifting was night and day better. Mine is still a jerk about going in to first when it's cold. But after its up to operating temp it shifts great!

As far as shifting into revers mine likes to give me trouble but if I go into 1st first it's slips right in. No idea why, I've gotten used to it.
 

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Discussion Starter #4
I used a Royal Purple syncromesh substitute when I first got mine. After a while I switched over to the actual GM syncromesh. Shifting was night and day better. Mine is still a jerk about going in to first when it's cold. But after its up to operating temp it shifts great!

As far as shifting into revers mine likes to give me trouble but if I go into 1st first it's slips right in. No idea why, I've gotten used to it.
When you say "actual GM syncromesh", does that mean the bottle actually had the GM logo on it? Or was it simply a gear oil "suitable for GM synchromesh", but not Royal Purple?
 

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Contrary to what I've heard, I don't have any trouble shifting from 2nd to 3rd. I just push up on the ball with my palm and it goes right into 3rd.
Is that speed shifting or normal driving shifting?

I find it hard to speed shift into third but normal shifting is fine.

Liberty Transmissions in Detroit apparently has a fix for the speed shifting issue. I don't need to speed shift but as you can see in my sig I have taken her to the drags.
 

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If its hard to shift when warm I'd be looking for a new fluid.
https://www.bobistheoilguy.com/forums/ubbthreads.php?ubb=showflat&Number=1114224
GM Synchromesh is what I found most highly praised when I put my used T5 in, @$15qt. That was before I learned it was made by Penzoil and patented, now owned by Shell so I guess you now say Shell makes it. Don't know if they still sell it in a yellow bottle but other brands like Honda and Mopar have sold it as their own so if ACDelco in your garage bothers you you have choices https://www.bobistheoilguy.com/forums/ubbthreads.php/topics/4872587/tremec-hp-mtf-no-more-atf
 

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Discussion Starter #7
Is that speed shifting or normal driving shifting?
With under 400 miles still on the drive train and rebuilt car, I've been taking it easy most of the time. Although, I have rowed through the gears quickly a couple of times. Thus far, I find it harder to shift quickly from 1st to 2nd than from from 2nd to 3rd. However, it seems to get easier the more I drive it.
 

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When you say "actual GM syncromesh", does that mean the bottle actually had the GM logo on it? Or was it simply a gear oil "suitable for GM synchromesh", but not Royal Purple?
Actual GM syncromesh. This is literally what I ordered.
https://www.amazon.com/ACDelco-10-4006-Synchromesh-Manual-Transmission/dp/B0026JK8FA/ref=sr_1_fkmrnull_1?keywords=ACDelco+10-4006+Synchromesh+Manual+Transmission+Fluid+-+1+qt&qid=1552674846&s=automotive&sr=1-1-fkmrnull

I wasn't happy with how mine shifted for the first few hundred miles or so. I had to pull my motor anyways to fix some stuff so decided to swap fluids once it was out of the car. Put it all back together and I was shocked at the difference. First is still a jerk when cold sometimes, and reverse still has no syncro but 2-3-4-5 are all butter now.
 
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