In the absence of the insider trader selling you oil futures on the basis of his insider knowledge, what would you do?

You'd buy oil futures at broadly the same price from someone else (maybe a worse price! Because the presence of the insider selling is already driving the price down). So how exactly do you lose?

The only people who lose out are those whose limit orders don't get filled because the insider outbid them. The counterparty benefits from trading with the insider.

This is completely ignorant of market microstructure. For big swaths of the market, most parties interacting with the market do so by trading with a market maker who ensures that everyone's trades clear immediately in exchange for a fee that covers the risk of getting caught offsides. If you're big enough, it makes sense to take that risk in house, but the risk and its very real financial cost remains. In both cases, trades that increase the risk increase the fee. So no, corruption is a tax and everyone pays the tax even though the mechanics have fallen through the rather large cracks in your understanding.

If you think this tax is de minimis, great, glad to hear it, let's put a government tax of similar magnitude in there and resume the peanut butter rations to starving african kids that DOGE cut.

I'd say the entire market loses out on insider trading except for the two parties involved in the insider trade. The insider trades take away a part of the profit margin that other good faith future providers need to justify the risk they take on by offering the futures. This leads to future providers needing to raise the prices of futures to remain profitable.