Hi Fif91,
I would use compression level 5 or 0.
By the way, 5 is the default level and there is a good reason for this.
The default compression is the tradeoff between size and computing (decoding) time.
Higher compression level only results in slightly smaller files.
Stephane wroteMy guess is that with a 4 cores CPU you won't have any trouble with the maximum compression.
The technical strength of FLAC compared to other lossless formats is its ability to be decoded quickly, nearly independent of compression level.
Frank