Define " worn out " and how old the cylinders are.

Some older ( 60's ) Girling cylinders had seals for hydraulic oil not brake fluid. But anything more modern than that will be OK with regular brake fluid.

If fluids are mixed, the rubber expands to 2x it's normal size rather than disolving.

For autocross use, the fluid you are using is fine. When road racing my FF ( Zink Z10C ) regular fluid was just fine, I'd flush once a year and call it a day. These cars are not that hard on brake fluid unless you have inboard rear brakes and full body work.( no air flow )

I haven't been on this list for a while but you should probably have a look. apexspeed.com is a very good formula car centric site.