GE and CFM use a centreline oil vent or breather system which exhausts through the cone fairing on the rear bearing housing. On earlier engines like the CF6-50 it was hidden inside the exhaust nozzle but modern designs tend to have protruding fairings that are obvious. This arrangement keeps the cowlings clean as any oil vapour exits in the engine exhaust stream. If you look at earlier RR and PW engines the cowls were covered in oil because they breathed through a gearbox vent that allowed any vapour to impinge on the cowls. The early RB211 was particularly bad as the breather outlet was about mid height on the left fan cowl and the oil slick clearly visible especially with painted cowls. RR eventually came up with an extension to the outlet to keep the vapour away from the cowls.