Oil in fuel

[ksmech11]

Got a 2011 prostar with a cummins 871. Just had in frame done 10k miles ago. Oil pressure was bouncing around from 11 psi to 20 psi at idle. Then at running be at like 30-35 psi and drop below 20 psi. driver shut down during unload. On way back oil psi stayed at 30-35 psi. I did a full service on it. Found what i believe to engine oil in fuel filter. Took sample and it is very dark. Some guys say this is normal. Im not sure if there is a coralation between the psi readings and the oil in the fuel. The tanks are clean. No algae. Any thoughts?