This is what I'd suggest - every year before summer, with Expella or electric extractor, suck out atf from the tube then fill with fresh. On 4th year, drop the pan and replace the filter. This way your trans will be at good shape.
(from tube extraction or drop pan, you can only get out of atf about 50%).
(from tube extraction or drop pan, you can only get out of atf about 50%).