Not much,keep it oiled with a fine oil,you'll have no problems
I would advise against oiling the sprue at all. I have never oiled mine and they are about 15 years old with no problems. Just keep the screw tightened properly and then give it a shot with a hammer handle, not the head!
Sometime during your casting sessions you will see the lead start to smear under the sprue plate. Tighten the screw and clean up the smear and keep going.
When tightened properly, the sprue plate should flop into position by turning the mould. It may need just a minor tap to completely go into place if the mould is new.