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hey guys! have a 2003 4l60e in my hummer h2. it has 240,000 km on it on the original transmission. i dont remember the maintenance history for the tranny between 2003-2015 i have no clue but i know around 2015-2016 ish? my mexchanic did a pan drop and put in some fresh fluid but i dont know if it was dexron iii or dexron 6. so now i want to switch out the fluid all 16 quarts or whatever thru the coolers lines but dont know what fluid to use. ive heard the saying that you should never switch to synthetics later on because it leaks, but i also read the 4l60 doesnt like synthetic as it is too slippery. i have no clue what the mechanic would have used 5 years ago, if dexron iii was still recommended for my truck then obviously that. but it seems like dexron 6 was released in 2006 ish and it superseded the dexron iii and is a semi/full synthetic as a minimum



now for the new fluid i am looking to use for 16 quarts, i dont know if i should try to find a regular non synthetic dexron iii which i dont even know if i can find, or just use gm's dexron 6? 

my problem is this:

lets say the truck has dexron iii in it and when my mechanic did the pan drop, he found dexron iii and put it in

theres also the possibility the fluid was changed in like 2008-2011 ish and dexron 6 went in and then in 2015-2016 when my mechanic did the pan drop he did dexron 6


if it has dexron 3 in it rn then i will for sure just put dex 3 in it again but if its dexron 6, then i dont know if i feel safe going down to a dex 3, but i also read the 4l60e hates synthetics which makes no sense since gm basically made a synthetic fluid for it


any 4l60e wizards here



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