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Hi Boris,
probably engineers wanted to make it easy (I'm an engineer, so I'm not so sure ;) ), however they couldn't because the hydrokinetic gearbox consists of two oil spaces - the gearbox itself and the torque converter. Gearbox case always has a drain plug, converter sometimes, but not in this case (rotational mass easier to balance, cheaper). Only way to flush the plugless converter is to use internal tranny's oil pump (WIS way) or external one (service sold in shops as "dynamic oil change") to push the oil through the circuit.
If gearbox is in such a bad condition that refuses to work properly on fresh oil then it needs rebuild anyway and mixing old oil with new one will not help either. Fresh oil is not an aggressive detergent, but some additives are, that's why I would not recommend adding anything to the ATF.
I know that you are talking about some rare cases (but heavily rumored on the Internet, because it's a failure) when after oil change gearbox lost some functions (slipping, lost gears, etc.). Usually those cases are related to very neglected boxes, typically: no regular oil changes at all until some malfunctions observed, then the change as a cure and more issues. Yes, in such case the fresh oil could dissolve some of the heavy debris and clog some vital parts like valve body. Additionally it happens that wrong oil is used what makes the issues even worse. But let's agree that it concerns to boxes that were maintained not properly. Many of this situations are related to cars, which manufacturers stated lifetime fill (like Volvo).
If the tranny is in a good shape then it works better on new oil. Mine has now ~300.000km and had two oil changes (at 160.000km - done by the dealer, so probably whole flush like in WIS; then at 270.000km done by me after buying the car, WIS method). Works good, definitely better after oil change.
So summarizing - best method is to change as much oil as possible to avoid contamination of new oil. This is the way! ;)
PS. Same situation occurs when changing oil in 9-5 engine - only 4 liters of 5,5 could be changed by draining the sump. The rest stays in oil cooler and in the pipes (over 25% !!!). So I'm draining the oil cooler and flushing it with solvent to prevent accumulating dirt during each oil change. Please note - this time engineer is not following the WIS ;)
posted by 80.50.141...
, Fri, 1 Jan 2021 10:57:07
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