There's a fleet fuel station nearby where I live, is it legal to use it if I sign up?

It has a min cetane rating of 47. You need to sign up for their card in order to purchase and use their fuel.

If I sign up and use it in my car, what's the worse that can happen?

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1998 Ford Escort ZX2 5 speed - 279,000 miles My Daily

1992 Mercedes 300D 2.5 202,000 - Pure junk
2000 Mercedes E320 Black - 136,000 miles - Needs repair

Don't forget to grease the screw and threads on the spring compressor.