the crank case breather system. You can choose to vent to atmosphere or return. The usual bi-product of excess oil blow-by is increased chance of knock. Did you notice if you were having issues with that? Not sure if it would show up on a wideband either, since technically the A/F ratio would be the same.
1993 Nissan 300ZX Slicktop (Oxford Gray Metallic) 1992 Nissan Skyline GT-R (Gunmetal Gray) - Long Live the Z and Godzirra! - "Love cars. Love people. Love life." - Mr. K.