In the kitchen, cooking oil, most often pressed from soybeans, rapeseed or peanuts; the southwest favours a dark, nutty roasted rapeseed oil. Stir-frying and deep-frying use plenty of it.
加油 (jiāyóu, literally 'add oil') is also used as an encouraging expression meaning 'Go for it!' or 'You can do it!' It was even added to the Oxford English Dictionary in 2018.
Left water radical (Kangxi #85 水 in its 3-stroke compressed form). Anchors 油 in the liquid family: oil is a viscous fluid. The radical groups 河 (river), 海 (sea), 汁 (juice), 汽 (vapour), 酒 (wine - also liquid). Three small downward strokes pictographically suggest droplets running down.
Right phonetic 由 (yóu) - supplies the sound directly without shift. Same phonetic family as 邮 (yóu mail), 釉 (yòu glaze), 抽 (chōu pull), 袖 (xiù sleeve), 宙 (zhòu cosmos). Standalone 由 also means 'from, via, cause' - purely phonetic here, no semantic carryover into 油.