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Hello all, new user here. I was wondering what transmission fluid to use in my 04 STi for synchro wear. I have the 5th to 4th grind at high rpm. When I’m cruising at 50-60 and want to downshift to 4th I get a grind even double clutching. I put a quart of Pennzoil Synchromesh in it with the remaining being Subaru fluid. My shifting has been smoother. The downshift grind is annoying and I’m thinking its probably shot the synchro. Any tips would be great. And also I tend to shift at low rpms around 2k-2.5k and I baby shift it. If I shift at 3k will it put more wear on my motor? I noticed that my shifts are smoother at higher rpms. I run Motul oil too. I’m tuned by TPG also. Is rev hang normal too? I put my clutch in and my rpms jump a few hundred rpms. Maybe tpg can tune it out? Sorry for the chaotic paragraph. Thanks!
 

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1) Don't lug your engine. it will be far happier near 3K than below 2500 yes I mean at cruise! downshift more, at least a gear to accelerate. Been DD an 05 for 7 years. I just realized how bad 04-05 gearing is. I have argued with members here that STIs are difficult to drive smooth. But, I just learned how much more user friendly the longer 07+ gearing is. In either case, for normal driving shift near or above 3K.

2) Shifting issues could be a clutch issue. How many miles on the car? perhaps someone else can provide a decisive way to determine between clutch ans synchros.

3) You can change to oil and hope it will help. many of us run Motul Gear 300, but any decent 75-90 will work. If the car is high mileage, the trans plan and clean the magnet. I was amazed at how much metal there was on mine - Ozs.
 

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Discussion Starter #3 (Edited)
1) Don't lug your engine. it will be far happier near 3K than below 2500 yes I mean at cruise! downshift more, at least a gear to accelerate. Been DD an 05 for 7 years. I just realized how bad 04-05 gearing is. I have argued with members here that STIs are difficult to drive smooth. But, I just learned how much more user friendly the longer 07+ gearing is. In either case, for normal driving shift near or above 3K.

2) Shifting issues could be a clutch issue. How many miles on the car? perhaps someone else can provide a decisive way to determine between clutch ans synchros.

3) You can change to oil and hope it will help. many of us run Motul Gear 300, but any decent 75-90 will work. If the car is high mileage, the trans plan and clean the magnet. I was amazed at how much metal there was on mine - Ozs.
I have 104,900 miles on the car and I just replaced the clutch 500-600 miles ago with a new pressure plate, 6 puck disc, throwout bearing and pilot bearing. I also ran Subaru transmission fluid. I saw online that grinds can be smoothed out with synchromesh fluid so I tried a quart of pennzoil synchromesh and it definitely smoothed them out more and feel crispier. I know the 04-05 transmissions have the bad synchros, but I am really annoyed as if I want to downshift at 40mph-55mph I get a bad grind and it pisses me off lol. Anything below 35mph and double clutching it goes in like a baby. Sometimes also when taking off in first it is very hard taking out of 1st and going to 2nd. Its very notchy/hard sometimes like it doesn’t want to come out of 1st and by this I mean my shifter doesn’t want to come out of 1st gear spot, I have to put a tiny force to take it out like its stuck, but sometimes its smooth like a baby going from 1st to 2nd. Maybe my shifter bushings or cable or shifter is bad? I’m confused but I’ve been shifting at 3k rpms now and its smooth up-shifting. Only reason I shifted so low was because I was babying the car and scared of aggressively shifting, but if 3k rpms is considered normal and not aggressive I’ll take the smooth shifts any-day. Also yes, shifting on these cars are so hard for it to be smooth. My mk3 supra was smooth like butter and had 200k+ miles on the tranny. I hope someone can just reassure me that my synchros are probably shot and recommend me a cocktail for it. I seen 1 quart of Pennzoil Syncromesh, 2 quarts of Redline Lightweight and rest either Motul or Castrol. I appreciate the response also, I was expecting no responses lol.
 

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I believe that worn synchros will allow play - but if you have pther sources of play it will be difficult to differentiate. How much play is there in your shifter? Though it's not going to tell us much unless you have aftermarket shift bushings. 100K is not a not of miles - except yes, I think there were some issues on the early transmissions.

New clutch, what clutch? Did you do it? Shifting was fine when first replaced? Recent work is always suspect no matter who does it . . .

I doubt very much a cocktail will fix it, but whatever the fluid, replacement is an easy first step.
 

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Discussion Starter #5
I believe that worn synchros will allow play - but if you have pther sources of play it will be difficult to differentiate. How much play is there in your shifter? Though it's not going to tell us much unless you have aftermarket shift bushings. 100K is not a not of miles - except yes, I think there were some issues on the early transmissions.

New clutch, what clutch? Did you do it? Shifting was fine when first replaced? Recent work is always suspect no matter who does it . . .

I doubt very much a cocktail will fix it, but whatever the fluid, replacement is an easy first step.
6 puck act and my shifting before was hard and my clutch blew actually and my throwout bearing snapped. I’m thinking my linkage cable is bad or something cause I had issues before.
 

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6 puck act and my shifting before was hard and my clutch blew actually and my throwout bearing snapped. I’m thinking my linkage cable is bad or something cause I had issues before.

I drove my 04WRX to 227K on the original clutch. The 325is before that went 299.9K on tit's original; clutch. Yet I had two ACT clutches fail prematurely (10K mi each ) in a row, and I had some difficulty shifting before they before they failed entirely. hat why orriginally mentioned clutch.

Linkage is a solid linkage. It can have play but is not likely your problem.
 

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Discussion Starter #7
6 puck act and my shifting before was hard and my clutch blew actually and my throwout bearing snapped. I’️m thinking my linkage cable is bad or something cause I had issues before.

I drove my 04WRX to 227K on the original clutch. The 325is before that went 299.9K on tit's original; clutch. Yet I had two ACT clutches fail prematurely (10K mi each ) in a row, and I had some difficulty shifting before they before they failed entirely. hat why orriginally mentioned clutch.

Linkage is a solid linkage. It can have play but is not likely your problem.
I adjusted my cobb shifter and also adjusted my clutch pedal height and shifting has been better, but still have a hard time getting the shifter out of 1st sometimes. Still also having the 5th to 4th downshift grind. Going to try the cocktail mix in a week or so.
 
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