I too use synthetic In my 08 duramax but the 400 friction plates and seals was not designed for that, there are companys out there selling the "wonder" fluids for the application but use at your own risk problems won't show up right away buy what I have seen from this was lip seals swelling causing them to go soft and in time a internal leak. I don't clam to be a expert on these but it seems I stay in the school of "hard knox" LOL. I learn something new EVERY day and it cost money to attend that school!!! Lesson learned?? Ha Ha.
The TH400 is a very robust transmission, in my opinion it is one of the best Turbo Hydros that was made, They will take more abuse than given credit for, We've used them in demo derbies with the coolers totally cut off and a steel loop from input to output, and have gotten them cherry hot to the point the pan gaskets are cooking, and after pulling them apart to use inother projects, they look decent.
They aren't meant for synthetics, the internal pressures are not high enough and the tolenences are not close enough to use synthetics, because they are just too slippery, they'll go for 200-300 k with proper DEX-3 changes and care.
In fact they shift better after rebuild using Mercon ( But Only After Rebuild Using Virgin Oil And Clutches!) which does not have as much friction modifiers in it, allowing the clutches to grab, instead of slip, Gm did allot of R&D to make these 400's shift like glass.
As far as the coolers you can do it a few ways, but I recommend you use it in conjuction with your radiator cooler, because you want some heat to keep the internals clean and the fluid from gumming up in the pan and servos, now I ran a TH-350 with the rad cooler totally out of the loop with just an RV cooler in front of the radiatir, but it was a 79 chev k-20 with an 1800 stall convertor and 40 in. mudders that generated enough heat.
Just My
