Joke played on those who are not mechanically inclined. 710 is OIL spelled upside down, therefore someone might be reading the cap upside down and read it as 710 instead of OIL. It is the same kind of joke as blinker fluid and muffler bearings.
Apparently this feller was in a local car parts shop when a woman walked in and asked for a seven ten cap. Two blokes behind the counter and our hero looked at each other and one said, "What's a 710 cap?"
She said "You know, it's right on the engine. Mine got lost somehow and I need a new one."
"What kind of a car is it on?" someone asked. She said a Ford.
"How big is it?" She makes a circle with her hands about 3 1/2 inches in diameter.
"What does it do?" She said she didn't know know, but it had always been there.
Someone gave her a note pad and asked her to draw a picture of it. She drew a circle and in the center wrote 710.
The blokes behind the counter who are looking at it upside down as she writes, fall about laughing and one goes and gets her an oil cap.