Is fluid change based on time or use?

[Maengelito]

i think the fluids are both time and use dependent. that being said, on the subaru, i try to stick to the safe side of the factory recommended intervals except with the oil. the suby says 7500 on oil except if driven hard, then 3750. i usually change it every 5k. the tranny/diff, i go about 15k (or every 3rd oil change for those that are math challenged). the coolant i like to drain and fill once every year or year and a half. its a lot easier than flushing and i figure if i do it more often, then i dont have to worry too much about flushing. i've been meaning to flush the brake fluid in the suby, but have put it off for about a year.

for the miata, i try to change everything except oil once a year. the oil i want to change more frequently. i've been trying to get on a schedule where i do 2 colder events on an oil change or one warm/hot event and all the associated street driving on an oil change. i also mix synth and dino oil because i change it more frequently, it keeps costs down a bit (i know, only a couple of $$) but the miata valves chatter a lot less with dino i've noticed. i've always run ATE superblue, but this last event at VIR was the first one with this miata and the PO used motul600. the brakes felt fantastic, so i might stick to leaving it in for the rest of the year unless i feel some sponginess.