Dont use oil unless it’s designed for it. Think of it as a soup pot with a cover (tight one at that).. I normally pack it with my food (take pork hocks, sauerkraut, potatoes, carrots and onions for example).. I pour enough water to come up to almost the top of the food (3/4 quarter of the way) and let it run for 40 minutes. When I say run, Once it starts rocking I start the timing and turn it down a few notches on the heat.. Enough to keep that rattler on top rattling..
One thing though, it may have a safety lock that wont let ya turn the cover off until it’s cooled. Quick cool way is to run the pot under cold water (you’ll know it’s cooled when ya take the rattler off and no steam flys out).

Edit: http://homecooking.about.com/od/foodhistory/a/pressurehistory.htm
http://www.gopresto.com/recipes/pressurecooking/index.php

OHHH.. And one important thing, you may want to replace the gasket if it’s been sitting for 20 years. Nothing expensive, and alot cheaper then having to sit there and scrub your stove.. (Most department stores, and places like Meijers and Walmart should stock them)…