High Mileage, leaky, 99 Corvette T-56 ATF fluid recommendation

[CJK440]

My kid just got a 150k mile 99 Corvette with a T-56. Its an earlier one with paper blocking rings (not compatible with most synthetics) but shifts good. It needs a fluid change as we are changing all. It slowly weeps ATF between the transmission and diff which is surely the output seal. It's a big job to replace so looking to reduce or stop it with a mechanic in a bottle for now.

I want to try something that could restore the seals performance but do not want to do something drastic and hurt syncro operation (no synth etc).

I am about to try Castrol Transmax High Mileage Automatic Transmission Fluid which has stop leak additives. Its for pre 2005 GM vehicles and meets Dex III specs. Seems like I can't go wrong.

Is there anything else that could work better? Maybe a regular Dex III with a more effective stop leak additive?

​

 

[ClutchDisc]

Maybe try Valvoline Maxlife ATF? I am not sure if it meets the spec for your transmission but if so perhaps it might help with the leak.

 

[rodinator1234]

Penzoil Synchromesh is what I ran in my 97 Camaro with a T56 with the paper blocker rings until I rebuilt it. I do not know if they still even make it.

 

[kebabaluba]

Just a bit of topic maybe but how does a C5 with 150k miles holding up? 150k is serious mileage for a sportscar...

 

[QwikKota]

Penzoil Synchromesh is what I ran in my 97 Camaro with a T56 with the paper blocker rings until I rebuilt it. I do not know if they still even make it.

They do but it's twice as expensive as the Valvoline synchromesh. I believe that's what I ran in my Viper T56 I had in a Dakota. I don't recall what the blocking ring material was made from though.

 

[Patman]

Just a bit of topic maybe but how does a C5 with 150k miles holding up? 150k is serious mileage for a sportscar...

There is a C5 in the Corvette museum that has 773,000 miles on it. For a sports car they are very reliable. I had my C5 for 9 years and it had 133k on it at the end. It only cost me about $2000 in repairs.

 

[2strokeNorthstar]

Just a bit of topic maybe but how does a C5 with 150k miles holding up? 150k is serious mileage for a sportscar...

Not for a Corvette that is maintained. They have more problems from sitting a short weekend drives.

 

[CJK440]

Just a bit of topic maybe but how does a C5 with 150k miles holding up? 150k is serious mileage for a sportscar...

It wad originally owned until last year. New england car some surface rust underneath, had to replace rear brake lines but it's a virgin unmolested fixed roof Coupe.

They do but it's twice as expensive as the Valvoline synchromesh. I believe that's what I ran in my Viper T56 I had in a Dakota. I don't recall what the blocking ring material was made from though.

I remember when pennzoil synchromesh came out and that it was a replacement for the gm stuff that was designed to play with brass blocker rings but more affordable. This transmission shifts fine and probably has its factory fill so it's rather stick with something that meets dex 3 specs.

Maybe try Valvoline Maxlife ATF? I am not sure if it meets the spec for your transmissionmission but if so perhaps it might help with the leak.

Although their synth dex vi seems more prominently available, they do have something that looks competitive to the castrol but at $13/qt vs $27/gal for the Castrol. I think I'm going to try the Castrol especially since they publish a tech sheet that shows it meets dex iii. For 2005 and older gms

 

[clinebarger]

Penzoil Synchromesh is what I ran in my 97 Camaro with a T56 with the paper blocker rings until I rebuilt it. I do not know if they still even make it.

They do.....It's also what I run in T56/TR6060 units.

 

[rodinator1234]

They do.....It's also what I run in T56/TR6060 units.

It worked really well in a T-56 for me.

 

[DannyM]

My kid just got a 150k mile 99 Corvette with a T-56. Its an earlier one with paper blocking rings (not compatible with most synthetics) but shifts good. It needs a fluid change as we are changing all. It slowly weeps ATF between the transmission and diff which is surely the output seal. It's a big job to replace so looking to reduce or stop it with a mechanic in a bottle for now.

I want to try something that could restore the seals performance but do not want to do something drastic and hurt syncro operation (no synth etc).

I am about to try Castrol Transmax High Mileage Automatic Transmission Fluid which has stop leak additives. Its for pre 2005 GM vehicles and meets Dex III specs. Seems like I can't go wrong.

Is there anything else that could work better? Maybe a regular Dex III with a more effective stop leak additive?

​

Probably your best bet in the link below... Start going through the reviews and yes here and there some did not have good luck but by far a huge majority did have success with this : https://www.amazon.com/ATP-AT-205-Re-Seal-Stops-Bottle/dp/B000NVW1LM