Why? Never heard that. I know DOT5 is incompatible. But you can mix DOT 3 and 4. I believe Castrol is DOT4. So how does it work when you initially bleed the SRF into the brake system that has non-SRF fluid? Some of the old fluid will mix.
2015 F34 328iX GT Glacier Silver
2011 E90 M3 Monte Carlo Blue
2004 E46 M3 Imola Red