These higher performance outputs for the ethanol are also caused by the higher oxygen content of ethanol fuel.
If I put higher instead of the higher, the meaning of the sentences will change?
I disagree with stangdon on this one. It can be written either way. "Content" has a non-countable meaning in this context (just means that ethanol fuel contains it).
If you're describing the chemistry of ethanol fuel, "higher oxygen content of ethanol fuel" can be just a chemical characteristic. If you are focusing on the specific characteristic that explains the higher performance to the exclusion of all other characteristics, you can add "the".
Either way works. In fact, you could ask the same question about "ethanol fuel" at the end. You can refer to it generically, without "the", or add "the" to refer to the specific fuel described earlier.
Leaving out the article makes the sentence look incorrect to this US English speaker. Content is a countable noun, and we're referring to a specific content here, so the is an appropriate choice.