Hello, wow, I am confused on whether to use Dextron III or ATF+4

I am a die hard AMSOil fan and have it in everything, but NEVER used it in the trans because my brother put it in his vehicle about 8 years ago or so, and his trans slipped because too slippery, so he pulled it out and went back to standard fluid.

Now that was 8 years ago, but I have no evidence of whether that has changed...

So brand do I go with??

I have a Trans shutter, so local family garage and trans shop is flushing trans fluid, changing filter and replacing torgue converter selenoid valves and adding the shutter additive (around $300, seems way to high). He says go with ATF+4 (ATF5 or 6 if not made for my older 1997 trans)

NAPA has Valvoline ATF+4 for $6.30 per quart x 14 quarts $89
CarQuest has Valvoline ATF+4 for $6.80 per quart
then other parts house has Wolfhead ATF+4 for $3.25, hald the price??? what the heck??? He says Wolfhead controls Penzoil and some tie into Castrol and Wolfshead is from PA's Oil City from early 1900's and they sold Wolfhead since 1922 when family owner started the parts store...

So do I use Wolfhead or go with AMSOil, but where is proof & reviews on both products???

Thanks so much!!!

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My Specs
* 1997 GMC Suburban 2500 5.6 TD 155k miles
* 3rd injection pump around 100k miles
* Installed Heath Diesel's Remote mount PMD & Turbo master Arm and their ECM/computer (woke this sleeping beauty up, WOW)
* Stock exhaust with cat punched out
* AFE airfilter
* AMSoil 15w40 used year round
* New block heater
* Morosso Oil Pan stick on heater from Jegs last year (baby gets hot!!)
* 265 all terrain tires, but thinking about Blizzak's if they are the best snow tire???
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