Maybe the following helps:
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For PLA support I only use Bambu Support W (its also currently in stock again)
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For PETG Support I use either PLA as support or Verbatim BVOH (but be careful: Do not use this for base support, only as support interface! It has very bad bed adhesion, so in the support settings, you choose your main material as “base” and BVOH as support interface / this setting should be used anyway in any case). The flush from any support to PETG should always be set at least to 700 to avoid contanimation of the PETG by supprot material. From PETG to support you can set the flush to 250 (which is fully enough)
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For Fiber Nylon etc. you can use ASA as support. There is also an intersted post here:
ASA as support for PAHT-CF Fiber Nylon
I hope that helps.