He is talking about a Hobbs switch. You'll want a normally open one. You will want to put a tee inline with your oil pressure gauge and hook it up there.
Here is one on ebay.
NEW HOBBS PRESSURE SWITCH 4 PSI NORMALLY OPEN - eBay (item 180305885345 end time Nov-18-08 09:27:33 PST)
Thanks Lancem I forgot to mention that.