before pouring silicone rubber,do I have brush the item with releasing agent,such as petroleun jelly.will cooking oil have the same effect?

It depends on the type of silicone you are using and the material you are casting.

I use a product called Pinkysil made by Barnes Products available in Australia.

http://www.barnes.com.au/addition-curing-silicone/pinkysil-putty-silicone-rubber-1612?search_query=pinkysil&results=6

It does not need a release agent for resin, plastic or aluminium, but if you want to cast rubber or wood, or other porous mediums, it is best to seal them before using it.

If you plan to do a two part mould, a release agent between halves is a must.

The main issue with brushing a release agent is build up and brush marks that will effect the casting quality. Also you will not know if the agent will react with the silicone. I have read of issues where the mould did not cure around the master due to oils used.

You probably should test on some scrap of the type of material your master is.

thank's for the reply