Best fluid for rebuilt 1971 400TH transmission?

[71CHEVYSHORTBED402]

I read that Mobil Delvac ATF is real good (link below), so I put a quart in the torque converter. That said, maybe it's not desirable for a 400TH.

I checked with the shop that built my transmission, and they suggest topping it off Dexron. They don't see an issue mixing it with a qt. of Delvac. But I read synthetic is best, even for the old transmissions because they run cooler and shift smoother too. Not sure about seals and that.


I'm somewhere between:

A. Finish topping it off with Mobil Delvac Synthetic ATF. Doesn't show as Dexron compatible, but according to this link "The ONLY Dexron III fluids to have a OEM licence are TES 295Fluids approved by Allison.".......... Apparently Dexron III is desirable for these old transmissions, because Dexron IV is rough on old seals. That's how I'm reading it anyway. http://67-72chevytrucks.com/vboard/s...d.php?t=523334

B. Top it off with Mobil 1 Synthetic ATF. Apparently it's somehow different than Mobil Devac, but at least I'd be mixing synthetics from the same company. Per Exxon this fluid is GM Dexron compatible.

C. ??????????........No I'm not draining the converter


Thanks

[SunSoaked]

Don't over think it. Choose one or the other and be done with it. Don't want to drain the converter, then don't.

[sweetk30]

i have a fresh 4l80-e to fill and my trans guy said fill it with dex 6 spec . better stuff these days over old stuff .

[71CHEVYSHORTBED402]

Originally Posted by SunSoaked

Don't over think it. Choose one or the other and be done with it. Don't want to drain the converter, then don't.

Dude, I overthink everything

[BigRandy]

I had a friend who ran a transmission shop for decades. He ran Type F in his El Camino because he said it gave slightly firmer shifts, and a firm shift = less wear on the clutch plates and used F in all GM 350\400 truck transmissions. He also like to stand on it now and then. It was fairly stock but that EC would fly.

I replaced the Dexron in my 400 with type F and can't say I noticed a difference. It shifts the same as far as I can tell. Certainly nothing harsh in any way. I'd use a quality brand type F but that's just me.

[77c10bowtie]

you should never substitute dex3 and dex6 for eachother in any situation, the older transmissions don't like dex6 because of the material used is the seals and newer dex6 transmissions will not like dex3, if you had it built use whatever your builder recommends if you built it yourself use whatever it had in it. generally synthetic oil doesn't make any difference in the older transmissions.

[truckdude239]

Originally Posted by 77c10bowtie

you should never substitute dex3 and dex6 for eachother in any situation, the older transmissions don't like dex6 because of the material used is the seals and newer dex6 transmissions will not like dex3, if you had it built use whatever your builder recommends if you built it yourself use whatever it had in it. generally synthetic oil doesn't make any difference in the older transmissions.

DEX 6 is backwards compatible now i probably wouldn;t use it in a used trans mission but a fresh rebuild i would in a heart beat

[truckdude239]

also i like valvoline maxlife full syn atf i used it in my 2002 sububran with 220k mile and it got rid of my cluck between 2 and 3 shift


https://www.valvoline.com/our-produc...ti-vehicle-atf

[71CHEVYSHORTBED402]

Apparently there are three different Dexron III (F, G & H). Back in the day our 67-72 took the fist issue, Dexron, no number. If you click on Applications here, it shows Mobil 1 Synthetic (not Delvac) is compatible with the original, Dexron II, Dexron III G & H, Dexron IV, etc. https://www.summitracing.com/parts/m...0/applications

Dexron Wiki: https://en.wikipedia.org/wiki/DEXRON

[77c10bowtie]

Originally Posted by truckdude239

DEX 6 is backwards compatible now i probably wouldn;t use it in a used trans mission but a fresh rebuild i would in a heart beat

that's what i was trying to say, i wouldn't swap fluids on a trans that's already running one type.
also some transmissions still arent backwards compatable, the allison 1000 5 speed isn't compatable with dex6 at all

[68Gold/white]

Dexron VI (6) is synthetic dexron. Used in all newer vehicles...

Dexron III is NON synthetic, and compatible with any of the previous Dexron fluids.

I do not know what Valvoline synthetic fluid is, I am guessing a Dexron VI compatible fluid???

The ONLY fluid Allison will allow to be used in their transmissions these days is called Transynd. It meets their rigid TES 295 standard.

IF, IF the Delvac fluid is TES 295 compatible, I would use that fluid, without doubt!!! AND since the converter has one quart already, regardless of what the Delvac fluid is, you need to fill the trans with it, and use it.

There are essentially 3 GM types of fluids.
Dex VI, the newest synthetic trans fluid used in all GM transmissions, except Allisons.

Dex III the older non synthetic fluid we have used for years. THe III desigination is just its last updated (that I know of) formulation. SO this stuff can be used all the way back to the 70's, when Dexron came out...

Transynd or an approved TES 295 fluid made by another manufacturer. This only recommended for use in Allison transmissions. BUT could be used in a fresh build where the trans has no other fluid in it.

Mixing any of these would not likely hurt anything, but I am sure is not something that should be done!!!

I would also use Dex VI (6) in a fresh build, it's synthetic, and way better than Dex III, BUT Dex 6 does NOT meet the spec of TES 295 for Allison transmissions

Questions?

I maintain 26 school buses, and 25 pickups, vans, and suburbans. Yes I have training from Allison!!!

[71CHEVYSHORTBED402]

Originally Posted by 68Gold/white

Dexron VI (6) is synthetic dexron. Used in all newer vehicles...

Dexron III is NON synthetic, and compatible with any of the previous Dexron fluids.

I do not know what Valvoline synthetic fluid is, I am guessing a Dexron VI compatible fluid???

The ONLY fluid Allison will allow to be used in their transmissions these days is called Transynd. It meets their rigid TES 295 standard.

IF, IF the Delvac fluid is TES 295 compatible, I would use that fluid, without doubt!!! AND since the converter has one quart already, regardless of what the Delvac fluid is, you need to fill the trans with it, and use it.

There are essentially 3 GM types of fluids.
Dex VI, the newest synthetic trans fluid used in all GM transmissions, except Allisons.

Dex III the older non synthetic fluid we have used for years. THe III desigination is just its last updated (that I know of) formulation. SO this stuff can be used all the way back to the 70's, when Dexron came out...

Transynd or an approved TES 295 fluid made by another manufacturer. This only recommended for use in Allison transmissions. BUT could be used in a fresh build where the trans has no other fluid in it.

Mixing any of these would not likely hurt anything, but I am sure is not something that should be done!!!

I would also use Dex VI (6) in a fresh build, it's synthetic, and way better than Dex III, BUT Dex 6 does NOT meet the spec of TES 295 for Allison transmissions

Questions?

I maintain 26 school buses, and 25 pickups, vans, and suburbans. Yes I have training from Allison!!!

Wow, I don't have any questions. You saved me some bucks too. $50 for that jug at the local. Cheaper on the web.