Monday, May 25, 2015 at 00:58
Peter - Here is the official API oil chart that outlines the specifications/features/uses of the varying classifications.
API oil chart
In essence, the API regularly upgrades oil classifications via testing for qualities and additives content.
Every few years, the API oil classification changes and many earlier API classifications become "obsolete" (in the API's view).
There's nothing to stop you using earlier oil classifications, an earlier classification just means that there is a superior oil available, with a later API classification.
Engine development has only progressed due to constant upgrades in oil types and oil lubricating abilities.
New oil additives are found constantly, and these new additives increase the abilities of the oil to lubricate, to neutralise harmful contaminants, and to hold neutralised contaminants in
suspension.
Long-chain polymers were a new additive find that excited lubrication and engine engineers in the late 1950's - which polymers led to multigrade oils - and an expanded ability for oil to lubricate over a wider temperature range.
Long-chain polymers can make up to 30% of the weight of multigrade oil - however long-chain polymers have no lubrication abilities in themselves - they only keep oil thin when cold and thicken it when hot, thus keeping viscosity stable over a very wide temperature range.
What has been happening in the last 25 years is that ever-tightening emissions laws have made engineers design engines so that instead of post-combustion contaminants going out the exhaust and into the atmosphere - they are being held in the engine during the combustion process, then forced down past the rings, into the oil.
This leads to rapid oil contamination with combustion by-products - particularly soot (carbon).
To counter increased soot levels in the oil, newer classification oils contain much higher levels of detergents than older classification oils.
The higher detergent levels in the newest oils can play havoc with older engines that have been run on a lower-detergency oil - by stripping useful carbon from ring grooves and seals and seal surfaces, that has the effect of increasing compression, and reducing oil burning and leaks.
The latest engines require the latest classification oils to cope with the specialised and high demands on oils created by variable-vane turbochargers (these have a tendency to gum up if the wrong oil is used), the latest EGR systems, exhaust gas after-treatment systems, and low sulphur fuels.
The best advice I can give, is to use the oil classification that was in use when your vehicle was new.
This will ensure that the oil classification, additives, and detergency, matches your engines requirements.
The API states that all newer classification oils are "backwards-compatible" (i.e., each new classification is compatible with the previous classification - but this is not strictly true, as many owners of older vehicles have encountered oil-burning problems and leakage problems as the older engines were unable to cope with the newer "slippery" additives and the much higher detergent levels of the newest oil classifications.
For this reason, many oil manufacturers have now started producing "oils for older cars" with lower levels of detergents and additives.
Cheers, Ron.
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