What to look for in an LS1 car...?

[BigMac76]

Yup, make sure it's cold when you get there. Piston slap and valvetrain noises get washed away when it's warm.

Oil pressure should be around 60 psi cold, 45-50 warm and under light load. Mine usually runs like this until the oil starts to thin out, then runs about 40-45 psi hot, and about 45-50psi under a moderate load, casual driving at highway speeds.