In "Stuart Little" by E.B.White, Stuart was having a conversation with the attendant of a gas station:
"Five, please," said Stuart to the attendant. The man looked at the tiny automobile in amazement. "Five what?" he asked. "Five drops," said Stuart. But the man shook his head and said that he couldn't sell such a small amount of gas. "Why can't you?" demanded Stuart. "You need the money and I need the gas. Why can't we work something out between us?" The filling station man went inside and came back with a medicine dropper. Stuart unscrewed the cap of the tank and the man put in five drops of gasoline. "I've never done anything like this before," he said. "Better look at the oil, too," said Stuart.
What does the last sentence "Better look at the oil, too" mean?