OK thanks - lots of info on this site. I guess my main question gets at if we could label all oil use as 'necessary, important, or marginal' how much would fall in the 'necessary' camp. Ill look into some old posts.
This is a misleading thing to look at. Let's take one extreme example, the people making a living selling gasoline at the boat docks at Lake Powell's remote Dangling Rope Marina.

To anyone who doesn't visit there it's pretty "non-essential."

To them it's their livelihood. If they didn't work there, they would need to work somewhere else, or else starve.

Pretty much every use of hydrocarbon fuels is "essential" to someone. It's all shades of grey. When does gray become black? When does grey become white? It's irrelevant.

What we must ask is the rate of onset of the shortage. If it is too abrupt then the economy will have trouble readjusting. If it is gradual, then there may be less pain adjusting.

Here's the post you want, Sasquatch.