I'd follow this advise.
Stay away from the interior surfaces of the moulds, the cavities and the flat mating surfaces.
If you do apply any oil to help remove rust and old dirt, clear the oil from the blocks before casting.

Sometimes it helps after that to 'smoke' the cavity of the mould so the bullet fills out better and releases from the block(s) easier when the mould is opened.

Some use a match to apply a thin soot film.
I use a candle flame.

I don't like using a match as it carries some phosphorus and other chemicals with it from the strike ignition typical of a match.
That can cause steel to rust and pit if left over the course of a couple days.

You only have to smoke the cavity once, not on every pour. Then perhaps every so often if the bullets starts to hang up in the block when dropped or they don't want to fill completely.