Motor oil is a key to your car, lubricating parts that mustn't grind together. And there are lots of potential signals to tell you if it has gone bad.

Oil analysis isn't meant for finding nuts and bolts in the oil pan-that's what magnets are for. Rather, ... and can enter the engine through loose connections or bad filters.

Fresh olive oil has a pleasant, herbaceous scent. If the smell changes and you catch a waxy, crayon-like odour, that's a clear sign it's gone bad. Some might even have a musty, damp wood scent. Before ...

Once, dipsticks were the go-to method for checking oil levels and finding out if your oil has gone bad. But as odd as it may sound, many cars don't come with dipsticks anymore.