Many “petrol heads” are very choosy about the oil they use in their cars and it is common to see threads on forums from new owners asking for opinions on “What oil is best?” for their particular model. In the United States, many enthusiasts have samples of their old engine oil subjected to laboratory tests which usually measure viscosity, particulate contents, metals content and the acidity/basicity of the oil.
These four parameters provide useful information about the condition of the oil, especially if the same data is available for a sample of the oil when it was new.
In general, as oil ages, the changes that you would expect to see are that the viscosity will begin to fall, although really old oil may well shown an increase in viscosity.
Particulate content and metals content will tend to increase as the oil ages.
The acidity / basicity of the oil will tend to go from starting out being basic, but progressing towards acidity. This change is measurable and is often used as a determinant as to when an oil change is necessary. Car engines oils are generally basic when new, these been quantifiable by laboratory tests designed to find out how much acid you would need to add to the oil to make it neutral. The results of such a tests provide a parameter referred to as the “Total Base Number” (TBN) of the oil.
As the oil ages, the TBN value will fall as the oil is continually exposed to acidic combustion products. One very simple criterion for deciding on the time for an oil change is schedule that as when the measured TBN falls to a value of 1/3rd of the value when the oil was new. A key issue here is that the oil should never be retained in use until it becomes acidic, as at that stage, it is also becoming increasingly corrosive.
Unfortunately the laboratory equipment needed to carry out such testing is quite expensive and needs a skilled technician to operate it, so the cost of having such analysis done is quite significant. From time to time, relatively inexpensive products have appeared on the market intended to enable ‘DIY’ TBN testing. It’s not clear to me that these have been entirely successful.
For anyone interested in engine oil performance or analysis, there is a wealth of information on the website “Bob is the Oil Guy” https://www.bobistheoilguy.com/forums/