It's all about context

Why is it that everything related to car maintenance sounds dirty?

I give you the following dialogue, which could either have been uttered by me when I dropped my car off this morning for routine maintenance, or perhaps come from a 70s blue movie.  You decide.

"I need a lube job."

"You've gotta pull out the dipstick to check the oil."

"Oh, I definitely need my tires rotated.  Check the tread on those bad boys."

"That's a dirty filter.  Better replace it."

"We'll take it on a test drive, don't you worry."

Okay, I may be exaggerating slightly.  But still. 

Don't get me started on plumbers.