Tax Subsidies for Oil Companies

In the context of the world that Republicans live, I get why they might be against eliminating tax subsidies for oil companies. Eliminate them and oil companies will pay more taxes. More taxes = bad in the GOP world. But really the way to look at this is when you compare oil companies to all other companies. There's a corporate tax rate and it ought to apply to all companies (with the same amount of revenue) equally. Except oil companies get tax breaks to make their taxes less for some reason, than say Walmart. So why is it that Republicans allow such disparate treatment? Oh yeah, they've got to protect their political donations.