99 F250 Transmission Fluid Type 4R100 ? Dex/merc? - Ford Powerstroke Diesel Forum
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post #1 of 5 Old 12-01-2019, 09:33 PM Thread Starter
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99 F250 Transmission Fluid Type 4R100 ? Dex/merc?

Hey I have a 1999.5 Ford F-250 ,I had my transmission rebuilt and when I put it back together I filled the trans with around 16-18qt of Castrol Transmax Dex/Merc,but recently I had a bottle laying around and I read the back and it stated Do not use in vehicles requiring Mercon V ,now I’ve searched and got all different results but since the 4r100 does require mercon V should I flush my trans and fill with Mercon V Fluid, when we had my trans rebuilt my the builder flushed my converter since it was only a year old and said it should be fine but I do get shift flare going into OD only if I’m half or more throttle and the flare turns into the truck feeling like it’s in neutral if I’m into the throttle hard but grabs once I let off, I only mentioned this because I’m thinking maybe it’s due to the fluid type I used, all the other shifts 1-3 and gears are perfect.the trans also has a BD valve body and truck now has about 25k miles since rebuilt.

Questions are is Castrol transmax dex/merc atf safe for 4r100?
What could cause shift flare in OD only under moderate to heavy acceleration?
Should I flush my fluid and put Mercon v in ?

Mods to truck are Stage 1 injectors,ats turbo housing,wicked wheel 2 ,bd wastegate ,Riffraff FRX & HPX,Idm Mod,Bd valve body, PHP Hydra W. Gearhead & Tw tunes.
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post #2 of 5 Old 12-02-2019, 07:32 AM
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The 4r100 requires Mercon, Ford changed the recommendation to Mercon V when they quit selling Mercon. Mercon rated fluid is fine to use in the 4r100.

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post #3 of 5 Old 12-02-2019, 07:50 AM
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Valvoline max life dexmerc and 99% of the other dexmerc is the closest oil to the original Mercon.

Do some research some reported odd issues after using Mercon v in the 4r100s. I just stuck with dexmerc and no issues. The slipping is mostly the tuning and or clutch packs etc. ask for tad firmer shifts and try it with a stock/lower hp tune

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post #4 of 5 Old 12-02-2019, 09:43 AM
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This has been a long discussed topic, your Owners Manual says "Do not use Mercon V" but,,, lots of people have done it and it's track record now doesn't seem to show there's an issue. And now that actual Dex III has been out of production for 10 yrs you're not going to find it anywhere.

Read the thinking from about MercV from 6yrs ago (middle of the page) -> https://www.powerstroke.org/forum/99...-now-what.html

I've come to assume that, now that our transmission seals are all over 15yrs old, the potential for whatever is was that MercV might damage, is now gone as they're "hard enough" to tolerate it maybe...

I agree with Justin, Valvoline Max Life is a good alternative.
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post #5 of 5 Old 12-02-2019, 04:22 PM Thread Starter
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thanks for the responses guys, i didn't get to check my post today so I ended up picking up Castrol Mercon v and and ran 19 quarts through the trans to try to flush it, the trans seems like it shifts slightly better than before, I'm looking into a new tuner to buy tunes from and maybe a JW valve body I'm hoping maybe that will cure the OD Flare issue(it hasn't happened since fluid change but I couldn't push the truck hard due to snow),thanks again guys
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