Xeno wrote:

Oil in the engine is undergoing a continuous breakdown process.
Actually, if the truth be known, it is the additives in the oil which
are breaking down. So, when dealing with oil change intervals, two
factors need to be considered; additive depletion and oil contamination.
These are like the proverbial piece of string - there is no hard and
fast rule on when additives are sufficiently depleted or the oil
sufficiently contaminated to warrant an oil change. Ditto for the oil
filter.

Yes. In most cases, the VI is breaking down faster than the base oil.

However, if you're driving an old car with a lot of blowby, you may find
that contamination of the oil becomes a problem before breakdown does.
On the other hand, if you're driving a new car in widely varying temperatures
you may find VI breakdown becomes an issue long before contamination.

When I was an apprentice, and later, a rule of thumb, backed by the
vehicle owner manuals, was an oil change every 5,000 km (3,000 mi) with
a filter change every 10,000 km (6,000 mi). This was the norm for the
average 6 cylinder vehicles (GM, Ford, Chrysler) of the day, 4 cylinder
vehicles had more frequent oil changes.

What has happened since then? Oils (and their additives) have improved
out of sight. Also, engine ventilation systems (PCV) have improved
vastly. This has extended the oil change intervals. My car, a Toyota,
has the oil *and* filter change interval *recommended* by the
manufacturer at 10,000 km (6,000 mi). So, the oil change interval has
been doubled but the oil filter change interval seems to have remained
static.

Yes, this is true. Also we have many cars which use an algorithm to
estimate the state of the oil given the time and driving conditions,
and which can more precisely estimate proper oil change intervals.

What seems to have been forgotten here is that the recommended oil
change interval will depend on the use made of the vehicle. If you do a
lot of short runs where the engine never warms up sufficiently, you will
experience greater wear factors and increased oil contamination
requiring shorter than recommended oil change interval. Will that impact
the filter change interval? Maybe. It really depends on the filter
capacity - the point at which the filter will block up and commence
bypass. A large filter of, say, 1 litre (1 quart) capacity might not
need to be changed more frequently. On the other hand, a small filter
with half or a quarter of the capacity might need to have a more
frequent change interval.

Bingo.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."