I use Castrol SRF:
https://www.amazon.com/Castrol-SRF-R...dp/B000JL760C/
You flush either when it needs it (i.e., you boiled the fluid at your last event (something that shouldn't happen unless you're overusing the brakes)), or when the club you're running with says you must. For example, my local BMW club requires a brake flush every six months, while the local Porsche club requires an annual brake flush.
If you don't flush before an event, I strongly recommend that you bleed them.
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2017 M240i: 23.8K, 28.9 mpg, MT, Sunroof Delete, 3,432#, EB, Leather, Driving Assistance Package, Heated Front Seats | Sold: E12 530i, E24 M635CSi, E39 520i, E30 325is, E36 M3 (2)
TC Kline Coilovers; H&R Front Bar; Wavetrac; Al Subframe Bushings; 18X9/9½ ARC-8s; 255/35-18 PS4S (4); Dinan Elite V2 & CAI; MPerf Orange BBK; Schroth Quick Fit Pro;
GTechniq Crystal Serum Ultra Ceramic; Suntek PPF