What does the last sentence "Better look at the oil, too" mean?

Question

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?

Answer 1

This question belongs in mechanics.stackexchange, not literature.stackexchange. :-)

In 1945, when this book was written, cars needed their motor oil filled up every so often, and when you stopped at a gas station to ask the attendant to fill your tank, you sometimes asked them to check the oil to see whether it was low and you needed to buy another quart (or in this case, another drop). I don't know the details ... this is something I remember my father doing when I was a kid in the 1960s.