the standard oil seperator lives on the front of the rocker cover up sort of up above the turbo. It is a little black box, oil air goes through it and the oil is seperated and returns to the sump, however, as the oil vapour comes out of the engine the pipe branches off, one way goes to the seperator and the other returns to your inlet duct, if you look at the back right of the rocker cover you will see what i mean. The standard seperator is probably a bit sh!tty by now so may not be working to well which means oil can settle in your inlet duct that goes to the turbo and oily air can pass through and affect the octane rating of the fule mixture.
A catch tank/better sepeartor will collect any oil and only return air back to the inlet duct, my wilkinsons non specific stainless steel container catch tank is vented to atmosphere through a breather filter so theres no chance of any oil returning to my inlet duct. It is said that its better to re-route a hose to the inlet duct as the air being sucked into your engine helps to draw out the oily air mine works ok so i'll leave it.
I've retained the standard seperator just gave it a good clean out.