Correct, Castrol or for that matter
any synthetic, will not play well w/ Dot 4 (or lower of course). Thus a complete flush and refill including clutch, is required when changing to synthetics (if system can handle synthetics). And yes, smart choice having ONLY ONE fluid in system at a time... unless emergency at which point a mix would be better than not enough
Personally, I run Castrol SRF and, knock wood, have had zero issues of breakdown during long sessions or a season. Never any issues previously either w/ ATE. In a season with ~ 4-5 bleed/flushes, ATE will be about break even cost-wise w/ Castrol SRF.
Seems you prefer ATE and had no issues so I'd stick with as it's a fine choice. Just order bulk a season or half season's worth from Linda (w/ some other stuff) and I'm sure she'll cut you a break on price & freight - she usually does. ATE has good shelf life (years) un-opened so you are fine w/ regards to storage.
Pick your favorite now and do a full flush (not just bleed) on all four corners plus clutch. Drive for a week or so and then do another complete 4 corner plus clutch flush. This should expel most mis-match remnants and you'll be good to go.
If you still have issues there's something else cooking and I'd start w/ brake booster or master cylinder...
Good luck
