If you are aiming to replace the common usage, what you really need is a word for “gasoline”. I’d make this a loan word: ñasolin or asolin.

For “gaseous substance” I would use an abstraction based on “air”, probably vistale (lit. “airyness”)

+Paul Strack that first one is 'petrol' around here:)

+Tamas Ferencz Good point. A loan word of peterol or peteroleum would be better than a loan word from “gasoline” (which was originally a brand name).

Sámen uman melme,
Peteroleum nése,
Rongo utúvien,
Kalkahonda nése.

😋

Petroleum means "stone oil". Why not ondomillo, then? Cumma for "gas" seems ok.

+Ицхак Пензев I favor using loan words for modern and/or non-Elvish concepts like peteroleum, computer or episcopo (bishop). I think translating the etymology is needlessly opaque.

For a loan word, another option is *petaroleon (petaroleom-) with short form *petarol. This handling of tr preserves order (unlike metathesis to rt) and sound (unlike unvoicing to ts) through syllabification of r which would develop the preceding vowel a (PE19/83,86); this also coincides with moving forward the final vowel in Latin ‘petra’ < Ancient Greek ‘πέτρᾱ (pétrā)’. The shift m > n accommodates Quenya phonology and coincides with AG ‘ἔλαιον (élaion)’ > L ‘oleum’. The adaption eu > eo preserves the disyllabic structure (since eu is a diphthong in Q) and would also be the result of aio ~ ayo > eo from the AG form (in which /é.lai̯.on/ → /ˈe.lɛ.on/ → /ˈe.le.on/ according to Wiktionary).

*petaroleon with short *petarol seems more adapted to Q than *peteroleum with short *peterol. The latter may be more recognizable orthographically; both look foreign to Q with so many vowels and no consonant clusters. There is also lámatyáve to consider.

If, instead of adapting to Q phonology, we only require a spellable orthographic borrowing, *petroleum with short *petrol can stand as-is. This approach seems common for this word in real-world languages.

+Paul Strack as for "computer", we have a long ago established nonwa. But in general, I agree with you.

+Ицхак Пензев I would not use nonwa “computer” because it is based on an outdated (Middle Quenya) phonetic development [tm] > [nw]. In Tolkien’s later writings it would have become [tw]: see PE19/85. You could in theory update it to notwa, but personally I’d just use computer

+Paul Strack I agree with your non-elvish concept of episcopo (I have no idea how to italicize it) since Tolkien himself had evandl, and evandilyon which mirrored "evangelical". maybe we could even have perester for priest (from Vulgar Latin *prester "priest," from Late Latin presbyter "presbyter, elder," from Greek presbyteros)

+James Coish episcipo isn’t mine. I got it from Shihali on the discord server. I do like perester, but I think perespíter might be better. FYI, to make italic put an underscore (_) on either side of the word, and an asterisk (*) for bold.

Minor correction: Tamas first proposed episcop so I’d credit the word to him, even though he left off the o initially

Thank you Tamas

Another correction: sp isn’t a valid Quenya combination, so it would either need to be perepsíter, or more likely going back to perester is a better option.