If you're planning to save & invest and retire say in 20 years, would you rather have a market that goes up +10% every year or a market that stays flat or negative for as long as possible and then has all the gains concentrated later?

This is how you should think about market downturns. A market crash is a blessing if you're still accumulating wealth. A "lost decade" would be just about the best thing that can happen to you if you're in your prime earning years.

Besides oil watch out for ferts and grains too.
Iran is a major supplier of ferts (Urea)
Qatar is the world's largest supplier of LNG which is used to make ammonia and then ferts.

Who sells a lot of grains? It's the ABCD of feeding the world.
Archer Daniel Midlands
Bunge
Cargill
Dreyfus

Not sure which of these are publicly traded.