Author's Note: MK's car died, and I wanted to write something involving cars to help cheer her up (it made sense in my head at least). And what happens to Cas in this story may or may not have happened to me at least once...

Disclaimer: These characters are not my property.

Ever since he'd returned to the bunker, Cas had taken to spending his spare time skimming through the vast collection of knowledge the Men of Letters had amassed. Some of it he knew already, and some of it was actually incorrect, but he was constantly amazed by how much these humans had been able to discern through guesswork and research. It was fascinating reading, all of it, and even though Dean mocked him for always having his nose in a book, Cas loved it. Sam said Dean was just jealous because Cas paid more attention to the library than to Dean. Cas hadn't been able to figure out whether or not Sam had meant this as a joke.

Either way, the library had become his favorite place, and while Sam and Dean slept (because they were human and needed their four hours - he understood that now, having been human), he would pour over book after book.

Sam had given him a notebook ("You're sort of a hunter now, Cas, so you should have somewhere to document cases"), and whenever he came across something that could be useful later, he'd jot it down. The notebook was actually unnecessary, since angels had perfect recall, but it had been a nice gesture on Sam's part, and Cas didn't want to reject it.

He also had a pad of Post-it notes that he kept handy. Those had been from Dean, after he'd caught Cas crossing out large sections of text for being incorrect ("Sam'll have a fit if he sees you marking up ancient manuscripts - how 'bout you use these instead?"). Cas still wasn't sure why Sam would care, but he did as Dean asked, and now there were pink Post-its affixed to certain pages of most of the books, stating "WRONG" in very neat block letters.

This morning, Cas was working his way through a treatise on 15th century exorcism, making frequent use of his Post-its. He was carefully writing out his fourth Post-it in ten minutes when a shopping bag suddenly landed on top of his work, causing his pen to go wildly astray.

"It's a beautiful morning, Cas - time to emerge from your books!"

Cas scowled down briefly at the now-ruined Post-it, but his irritation only lasted a moment. It was hard to stay mad at Dean for very long. And it was only a Post-it.

"Good morning, Dean."

As he glanced up, Cas realized that Dean must be on his way down to the garage to work on the Impala, since he was wearing his grease-stained coveralls. A quick peek into the shopping bag confirmed that it held some sort of car-related items, though he wasn't sure what purpose an "Oil Filter" served, or why you would need two of them.

Picking up the bag, he went to hand it back to Dean, but Dean just raised his hands and stepped back.

"Nope, you're carrying the filters. I'll meet you in the garage."

Dean turned to walk away, but Cas was more than a little confused now. He did enjoy watching Dean work, but Dean usually found his stare unsettling ("Damn it, Cas; I can't concentrate when you're staring like that! Don't you have some ancient Enochian runes to decipher or something?"), so this was new.

"You want me to watch you work?"

As Dean chuckled, Cas decided that he would never fully understand humans. He hadn't meant to be funny, but Dean seemed to be amused.

"No, I want you to do the work, Cas. You got a car now; you gotta learn to take care of her."

A quick flash of panic crossed Cas' face - he'd taken one look under the hood when the car had inexplicably quit on him before, and the maze of hoses and odd-shaped metal parts had been overwhelming.

"Dean, I'd rather not…"

But Dean just grinned down at him and dropped a reassuring hand onto his shoulder.

"Don't worry - we'll start with something simple."

Cas nodded slowly. Simple was good.

"You might wanna lose the trenchcoat, though. Oil stains are a pain in the ass to get out."

Five minutes later, Cas was staring into the depths of his car, wishing fervently that he still had his wings. Dean was gesturing animatedly with one hand, pointing out various components and rattling off their purpose. His other hand rested on Cas' shoulder, and it was the only thing keeping Cas from turning around and walking out, as he tried to absorb all the new information coming his way.

"...and that's about all you need to know for now, so let's get started. First things first, you're gonna need the oil pan from over there, and the socket wrench should be in the toolbox…"

With Dean's guidance, Cas managed to loosen and remove the drain plug without much trouble, and as the black gunk poured into the pan, a smile started to slowly make its way onto his face. Maybe cars weren't as incomprehensible as he'd thought.

Once the old oil stopped dripping out, he carefully replaced the plug, and then, after Dean demonstrated how to work the special wrench, he squirmed into position to remove the filter. The bit with the wrench went smoothly, and he was just starting to feel a little bit confident in his abilities when it all went wrong…

As he loosened the filter by hand, oil began to leak down over the sides, coating the filter in an extremely slick film. And seconds later, as he twisted it that last turn, the filter shot straight out of his hand and down into the very full oil pan. There was a mighty SPLOOSH, and Cas suddenly found himself covered in nasty, slimy, black goo. It was in his mouth, up his nose, all over his face and hands, soaking through his shirt… It was awful.

He came crawling out from under the car immediately, spitting out black nastiness and trying to wipe it away from his eyes, silently cursing the car to an eternity in the depths of hell.

Dean took one look at him and practically fell over laughing. It wasn't helpful.

"Dean, this is not funny."

After several more seconds of hysterical wheezing, Dean managed to pull himself together and hand Cas a clean rag.

"Sorry, it's just… you look… you look ready to smite the poor car…"

And Dean dissolved into laughter once again, while Cas attempted to remove what oil he could and salvage his remaining dignity. When he caught a glimpse of his reflection in the side mirror, though, he couldn't help but smile as it occurred to him that Dean had a point.

"Perhaps I should - Crowley may have done something to it."

Dean's grin grew even wider.

"Was that a joke?"

Cas simply ducked his head and grinned in return, then tossed the dirty rag in Dean's direction.

"You should hope it was, since you're going to finish this while I go clean off."

As he headed quickly for the door, another smile flitted across his face as Dean's muffled cursing told him that Dean had just realized that there was oil everywhere under that car, pooled on the floor, dripping from the undercarriage… By the time Dean was done, his face and hands were going to be just as black as Cas' were. Maybe there was some justice in the world after all.

But one thing was for certain - Cas was getting rid of that evil car, preferably today.