I don't tow hardly at all. However, these transmissions or more-so the torque converter I believe, have an issue with bad fluid. Or, maybe it's the original fluid the trucks come with. Many complain of shuttering transmissions. Generally, that goes away when one changes the fluid, either doing just a drain and pan drop w/ filter like myself, or a whole system flush.
You are correct about the 8 speeds shuddering. But, the 6 speed also has it in some instances. Primarily because the torque converter is failing typically. There are accounts of this on here (search for them) and on the Facebook groups. The 6 speed is more trouble free than the 8 speed but not without occasional problem children.
I installed a deep PML trans pan and elected to go with the Mobil 1 LV HP "blue label" synthetic fluid used for the 8 speeds. All I can say is the 6 speed loves this fluid. It shifts super smooth and a bit quicker now. Looked at the specs of this fluid, it has a higher viscosity index than Dex 6 and is thinner when cold than Dex 6 but does not thin out as much as Dex 6 when hot. The viscosity difference is not that great. Anyway, while technically not Dexron 6, the LV HP fluid is working great in my 6 speed.
i have used oil analysis for ovr 30 years now.i used to use DA SPEED SPORT OIL AND THEY GAVE YOU A FREE ANALYSIS WITH EACH CASE. i dont know if they even make the oil anymore but i thought i would share with everyone how important it is: i sent a q966 ferrari v12 engine to detroit to a man( terry myr) who with a team of other GM engineers rebuilt italian exotic engines in their spare time. i had him put everything new in he brought new borgo liners and pistons back from italy ,cam chaine,etc,etc . the motor was sent back and ran perfectly. my analysis of the oil kept coming up over the limits moisture content but it was not milky as water in the oil looks usually like milk. i instantly knew there was something wrong. i live in florida so i insisted he ship the car to detroit and back . i felt like it was not my mistake thus swallowing the labor of taking the motor out as well as reinstalling it. in 1979 i spent $14,000 on the motor( equal to approx $50,000 today) sure enough they found the problem in installing one of the new liners there was not a 100% seal at the bottom of the liner( was not driven in the block all the way.if i remember right, it was about 1 mm from proper sealing. they made it right and never had another problem with the engine. i think terry is still in buisness and would recommend the company as he fixed it with no fuss. about 6 months later i restored a classic mercedes 200 diesel for my brother to commute to college with. i had the motor built by the best local machine ship in the area. once again found high levels of moisture in the oil analysis. had them tear it down and they found a leaking new head gasket.they redid the motor and never had another problem. IF i did not have the analysis i would have had 2 very expensive engine failures once the engines were past warrenty repair, $20,000 worth of engine overhauls( in late 70's money WAY more today ) buys ALOT OF $30 analysis's.i do the same thing on my body called functional medicine i have the boston heart institute check an in depth analysis of my blood twice a year. its amazing how much they can tell you much like the oil analysis!!
