The noun ochomilused in information on mountaineering, is written in a single word, without quotation marks or italics.
However, it is common to find this word written in different ways: «He reached a height 25 years ago when he climbed his first eight thousand in Pakistan», «Two climbers search this massif for the first winter ascent to an “eight thousand” in pure alpine style» or “This is how she became the first woman in history to access the fourteen eight thousand of the planet.”
The Dictionary of the Spanish language collect ochomilin a single word and round, meaning ‘in mountaineering, a mountain 8,000 m or higher’. On the other hand, according to the New grammar of the Spanish languagethe nouns and adjectives that end in ele, if they are not preceded by another consonant, form the plural with -es: ochomiles.
So, in the examples above, it would have been appropriate to write “He reached the top 25 years ago climbing his first eight-thousander in Pakistan”, “Two climbers search this massif for the first winter ascent of an eight-thousander in true alpine style” and “So she became the first woman in history to access the fourteen eight thousand of the planet ».
Finally, remember that, if you are not using the noun, but the numeral, it is written in two words: «The experience, defined as “wild” by Txikon, took them up to eight thousand meters from Manaslu», and not “… to eight thousand meters from Manaslu.”