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Uh...
It's all unleaded, it's illegal to run leaded gas in most places.
As far as regular versus premium goes, it's based on the octane rating (87, 89, 91, 92, 94, etc).
Engines that make more power, heat, and higher compression (turbo/supercharger, nitrous, high compression NA, etc) will experiencing pinging and knocking, which will start to break things.
Premium gas won't do anything for most economy engines, but most modified or high end cars require it.
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