use Kroil or PB Blaster. those are the best for loosening bolts. WD40 is pretty weak, and Liquid Wrench is better but nowhere near as good as PB Blaster (Kroil is better still but expensive and not widely sold in retail outlets).

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2005 Mercedes C230k sport sedan, 6 speed
1987 Porsche 924S - 968-engined track car