Note the color of the engine oil on the left is black and dirty. I guarantee if you smell it, it will smell burnt.
This is what happens to oil when it is overheated. It becomes black and loses its ability to cool and lubricate
the engine. The oil on the right indicates a healthy engine. The oil is caramel colored, clean, and translucent. If
you were to smell it, it would give off fresh, petroleum-like smell.
Consistency
The engine oil should be smooth and slippery to the touch. If it’s gritty and
doesn’t feel slippery, change it because it’s dirty or viscosity breakdown has
occurred, rendering the oil ineffective in lubricating and protecting your engine.
The presence of metal or plastic (some engine parts are made of plastic) in the
oil means that there’s internal engine wear that needs to be dealt with.
Smell
If the engine oil smells burnt, it should be changed. This indicates that the engine
was overheated. Leaving burnt oil in the engine causes engine damage due to
inadequate lubrication and heat transfer. A great way to determine internal
engine condition is to have an oil analysis done; this is much like having a
doctor analyze your body fluids. In such an analysis a vial of engine oil is taken
from your engine during an oil change. It is then sent to an oil analysis lab for an
evaluation of its contents. A lab can tell if there’s excessive wear material,
engine coolant, acid, or anything else that would be detrimental to your engine in
the oil alerting you of potential problems. One oil analysis lab that comes to
mind is Blackstone. There are others, just ask your car dealership or repair
facility who they use.