And you can guess who usually pays for that.

What happens, sometimes – and why?

One of the most common things is that the "technician" – who may have very little training or is just in a hurry, because these shops make their money on volume; i.e., on getting as many oil changes done per hour as possible – adds too much or not enough oil to the engine after draining the old oil. The reason why this happens is because unlike you or me adding fresh oil quart by quart (or via a jug that holds a gallon) most quick-lube places transfer the fresh oil from a drum using a gun that pumps the oil back into the engine. And this works just fine if the technician is paying attention – and has checked to make sure exactly how much oil he's supposed to pump into your engine before pumping it in. And – after he's done – double checking to make sure he did in fact enough and not too much (or too little).

This is why you ought to check to make sure the level is correct – before you leave the shop's parking lot. Is it annoying to have to do this? To have to pop the hood and pull (and check) the dipstick? Sure, of course. But it's less annoying – and potentially a lot less expensive – to make sure the level's correct before you drive off the lot and find out later that the technician put too much (or too little) oil inside your engine. Either being bad for your engine and possibly very bad, if you drive around for long enough that way.

Another good thing to check before you leave the shop's parking lot is whether anything's dripping that wasn't before. If it wasn't, then it shouldn't be. If it is, then you might have either of two possible problems – one of them serious. The first of the two is that the technician over-tightened the new oil filter. This can over-compress the gasket between the filter and the engine, resulting in leaks. It can also make it a pain to get the filter off next time.

The other source of drips is a drain plug bolt that was cross-threaded stripped when the technician re-installed it. When this happens, the technician may not want to tell you he did it – for all the obvious reasons. A cross-threaded bolt hole can usually be repaired, but it involves time and some skill to do that. It may require replacing the oil pan and that's not cheap and guess who'll be paying for that if the technician owns up to damaging it? So he might just put the drain bolt back into the damaged hole and hope it holds – for just long enough to establish plausible deniability. If the bolt works loose after you drive away and you lose all the oil in the engine – and end up needing a new engine – the shop may disclaim any responsibility and leave you holding the bag.