Several parts combine into one character.

Phonetic-semantic compound: 氵 (water-radical) + 由 (a phonetic). The encoded meaning: "the liquid that flows like water but isn't water" → oil. To ancient observers, oil was a "kind of water" that pours and flows, hence the water radical. Identical across 繁體 / 新字体 / 简体.

Mandarin: yóu, rising 2nd tone. 油 covers cooking oil, petroleum, paint, and slick textures: 石油 (shíyóu, petroleum — "stone oil"), 汽油 (qìyóu, gasoline — "vapor oil"), 食用油 (shíyòngyóu, edible oil), 酱油 (jiàngyóu, soy sauce — note: not actually oily, but called "sauce oil"), 油画 (yóuhuà, oil painting), 加油 (jiāyóu, add oil — both literal "fill the gas tank" and figurative "go for it / cheer up!"). The slang 加油! (jiāyóu) is the most-shouted Chinese sports cheer, equivalent to "Go! Go!" in English.

Japanese: on-reading ユ (yu) — 石油 (sekiyu, petroleum), 灯油 (tōyu, kerosene / heating oil — used for the kerosene heaters common in Japanese homes), 給油 (kyūyu, refuel), 醤油 (shōyu, soy sauce — note: same compound 醤油 as Mandarin 酱油, though Mandarin uses different reading). Kun-reading あぶら (abura) is the everyday word — 油 (abura, oil), ごま油 (gomaabura, sesame oil), サラダ油 (sarada-abura, salad oil), 油っこい (aburakkoi, greasy / oily — used for food).

醤油 (shōyu, soy sauce) — Japan's essential condiment — uses this character. The Cantonese romanization "soy" (from 醤 = sauce + 油 = oil) is what gave English the word "soy" in "soy sauce" / "soybean".

Mandarin's 加油! (jiāyóu) — literally "add oil" — has even entered English dictionaries (Oxford added "add oil" as an entry in 2018) due to widespread usage among Hong Kong English speakers.

Memory aid: water that flows differently — oil as a "second water".