I know old WWII prop planes used oil, but do modern day jets use oil? I looked at the blueprints of a few jets and couldn't find any oil tank. So do they?

Modern jets do use oil but not in quite the same way as piston engined planes do. its usually found in places like the gearbox. jets as you probably know have in theory only one moving part so the need for oil is far less than a piston engine.