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Yes, mixing old fluid with new one is bad - it degrades new fluid. It's obvious. Of course in your case when you are refilling the tranny so often it simply doesn't matter (it's just like mixing new one with new one). However if you change fluid after 100.000 miles and 10 years of service then "not to mix" matters a lot. IMHO it's simply not necessary to change the ATF too often, because, as I said before, the environment in the transmission is much more oil friendly then in engine. Changing too often is simply waste of oil, time and money (not to mention the environment). It's similar to changing brake fluid each 6 months instead of 2 years without any actual reason. It will not help technically, but only to keep your mind in peace. Unnecessary waste is the cost of such thinking.
Factory sealed transmissions (and other "unrepairable by design" things) are typical solution to sell a new thing instead of keeping old one longer than manufacturer wants. Nowadays each part of the vehicle is designed to last for certain amount of time. For a part supplier = just to pass the OEM specification. If you've designed a part which can survive twice the spec you will be asked to redesign it to make it cheaper. I worked many years in automotive, so believe me - it's just like that. Look at manual transmissions. In 90's each manufacturer specified the oil change interval for about 60.000km (Saab 9000). Later they did not recommended oil change at all (Saab 9-5). Was the tranny construction significantly different? No. It was only claimed that it is due to new magnificent synthetic oil, what was only partially true. Synthetics really lasts longer but not forever. Plus there are additional reasons to refresh fluid, like to flush out the particles (magnetic plug capacity is limited). But for a given time/mileage of typical first ownership it was fine ("fine" criteria for car manufacturer: failure probability peak over warranty). To be sure that user will not outsmart the manufacturer we will delete the plugs. Simple and effective. Usually users blame engineers for such design evolution - yes technically they did it but not driven by themselves - they were told by management to do it (three plugs less - we will maximize profit!).
PS. Hey, absolutely no other books than SAE journal in the childhood? ;)
posted by 80.50.141...
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