I have asked some very knowledgeable people about why HydraMatic decided to redesign the lube circuits on the later 4L80 transmissions, nobody has any really good explanations. The original 400, and the early 4L80, shared basically the same design starting in 1964. If it’s worked for 50 years, why change it?! In my experience, the only time you see damage to planetary gearsets or excessive bushing wear is when the trans is either run low on fluid, the cooler lines are kinked or otherwise restricted, flat towed for long distances with the engine off,or the filter gets plugged up. There are a bunch of dimensional changes to what looks like identical parts between the early and late model trans. The ATSG book says the changes were made to improve durability. How do you improve durability to a transmission that will easily handle twice it’s rated capacity? The torque capacity of the 4L80 is rated at 440 by HydraMatic, that’s ridiculously conservative. I have no idea what torture test process they used to come up with that number, nobody I know would hesitate putting twice that to any junkyard unit in decent shape. The ultimate weakness in any 4L80 is the overdrive sprag. In any gear other than overdrive, all the engines power has to go thru that relatively tiny sprag. Sonnax has developed a add on valve body to engage the overdrive brake clutch to help the sprag, but even with all that the 4L80 will never be as strong as a 400. I wouldn’t put too much into the belief the later 4L80 are better. The parts are basically the same, what is different is related to the center lube modifications.