My Diablo is giving me a P0171 System too lean (bank 1), also I cleared my codes from yesterday I had a misfire code.

First code sounds like a fowling O2 sensor?

2nd, oil from the blow off gets all over the RT valve cover and down around the plugs. (Catch can being installed soon)

 

#1 yes sound like an O2 problem. Make sure the flange connection from manifold is not leaking before the O2 sensor. It can suck air past O2 sensor and make it throw a code. Or your O2 may have just died.
#2 it is possible to cause a misfire if liquid gets past the plug boot. Earlier modulars are famous for water getting in there and causing misfires.