Since lubricating engine components – reducing friction for smoother running and optimal efficiency – is only part of what engine oil must do, it needs assistance to fully protect an engine. That’s where the additives come in.
One of these additives is detergent. Not the type of detergent we’re familiar with in a domestic context but one containing a dispersant additive that suspends contaminants and combustion by-products in the oil instead of allowing them to settle out onto engine surfaces as sludge.
That’s why your oil may look dirty when you check it – it’s doing its job.
And that’s also why you can’t tell when it’s time to change the oil simply by looking at its colour.
The oil is designed to prevent these corrosive contaminants from being deposited on engine surfaces, where they can cause piston rings to stick and oil-pump screens to block.
The oil filter helps by removing suspended abrasive particles as the oil passes through the filter, but an oil filter can’t correct the effects of fuel dilution and other liquid contaminants, like acids, in the oil.
No matter how good the oil and the oil filter, eventually they can no longer do their jobs effectively. That’s because with time and distance travelled, the oil’s additives get used up, allowing the oil to degrade (oxidise or thicken). At that point the oil must be changed, before sludge and deposits build up on engine surfaces, and when the oil is drained, the contaminants are removed with it.
Obviously, oil should be changed before it is contaminated to the point where engine damage could result. Yet it’s unlikely that the individual motorist will be able to determine this critical point. For this reason, automobile manufacturers recommend oil changes at a specific time or distance interval, whichever comes first.