oleum

Oleum, olei is a second declension neuter, so the nominative and accusative are the same and the plural olea is rare - oil is a mass, and Latin treats it as one. It is a loan, borrowed from Greek elaion before the Romans had olives of their own, and the tree and its fruit stayed separate words: olea and oliva.

In classical use it is the household staple - lamp fuel, cooking fat, and the stuff athletes and bathers rubbed on. In the ecclesiastical texts that make up much of this corpus the word has narrowed and risen: oleum is the consecrated oil of anointing, chrism, the oil of the sick. Keep the two senses apart when you read. English "oil" arrived through Old French from oleum itself, and "petroleum" is just rock oil.