the crank case vent on the 7.3 is in the intake tube before the turbo. as the turbo sucks air in it will also suck in the oily air from the crank case vent, over time the intercooler pipes and the inside of the cooler will get a light coating of oil on them. if you had one or more lose intercooler pipe clamps it would blow that oil out.
as far as running better, now that the clamps are tight you are not loosing boost out of the system anymore.
everyting they said and the symtoms you had sound right