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Conventional engine oil filtration...

Thanks a lot to Jim Elkins of Jackmaster Oil Filters at who provided me with a lot of authoritative information on engine oil for this section based on his depth of experience and expertise in this area.

Conventional engine oil filtration, which is used on virtually all production cars, involves the filtration of all oil with an in-line filter before that oil reaches any of the moving parts of the engine.  This filter is called an ‘in-line’ filter because all oil in the engine passes through it after having been sucked up by the oil pump from the sump and before being distributed throughout the engine by the force generated by the oil pump.  This in-line filter is the cylindrical engine oil filter that screws on to the outside of your engine.

Conventional engine oil filtration has very limited effectiveness…

Most of these in-line filters (also known as ‘full-flow’ filters) have what is known as a bypass valve.  If the filter gets clogged with contaminants such as metal particles so that the rate of flow of the oil through the filter is reduced (risking lack of oil supply to lubricate the moving engine parts), the bypass valve opens up to allow oil through and to ensure a continuing supply of oil.  However the down-side is that the oil that is allowed through the open bypass valve isn’t filtered, because it’s been diverted around the clogged filtration material.

Because it’s far from an ideal scenario that the filtration material becomes clogged to such an extent that the supply of oil is threatened and the bypass valve opens, the full-flow filter is not designed to filter to such a fine level that the opening of the bypass valve becomes a common occurrence.  A typical full-flow filter containing paper filtration material removes the following percentages of different-sized contaminant particles after a single pass through the filter:

  • 40% of 10 micron particles
  • 60% of 20 micron particles
  • 93% of 30 micron particles
  • 97% of 40 micron particles

(One micron is equal to 1/1000 millimetre.)

Therefore, a typical full-flow filter can be relied upon to remove the overwhelming majority of all particles larger than 30-40 microns in size.  However, considering that the minimum clearance between moving parts inside the engine is 25 microns and less, full-flow filtration doesn’t provide the kind of protection required to minimise wear inside the engine.  Wear increases as the size of the contaminant particle matches the size of the clearance itself.  Most full-flow filters will let a significant percentage of contaminant particles sized between 25 and 30 microns through unfiltered.  It is these particles which pose a risk to clearances such as between the bearings and the crankshaft, which are often 25 microns (1/1000 inch) in size.

More importantly, many clearances are even less than 25 microns (e.g. cam lobes to valve lifters or piston rings to cylinder walls) and will be adversely affected by unfiltered particles less than 25 microns in size.  Clearly, a typical full-flow filter will let through a significant percentage of contaminant particles sized less than 25 microns which cause noticeable engine wear in these cylinder head and upper-engine components that have such fine clearances.

Conventional engine oil filtration necessitates regular engine oil changes…

In order to minimise engine wear when relying only on a full-flow filter, the engine oil must be drained and replaced with fresh oil before it becomes excessively contaminated with wear-causing particles and other contaminants such as water, which the full-flow filter fails to remove from the oil at all.

The ideal service interval for engine oil which is only filtered using conventional full-flow filtration is about 3 to 4 months or 5,000 kilometres (3,000 miles), whichever comes first, depending on whether mineral oil or synthetic oil is used.  Synthetic oil can remain in the engine for longer than mineral oil due its greater resistance to degradation.

Even if little distance has been travelled, if the objective is to minimise engine wear to the greatest extent possible, the oil should still be changed after three to four months.  This is because water accumulates in the oil on a daily basis in proportion to the number of times the vehicle is started when the engine is cold, irrespective of distance travelled.

By changing your engine oil regularly, you can ensure that you get the maximum life out of your engine.  The objective is to change the engine oil before it gets dirty enough that it’s causing wear and tear on your engine, which is an uncomfortable thought.  Failing to change the engine oil regularly is one of the leading causes of premature engine failure which results in huge expense and inconvenience to fix or replace the engine if the vehicle is going to continue to be driven.

From my own experience, I’m confident that when relying only on full-flow filtration, it’s only by changing the oil every 3 to 4 months or 5,000 kilometres (3,000 miles), whichever comes first, that engine wear can be minimised.  If you still decide to use oil change intervals greater than four months (e.g. six months), it’s wise to replace the full-flow filter with a new one halfway between oil changes (i.e. after three months).  This prevents the full-flow filter from becoming clogged with contaminants which causes the bypass valve in it to open and allow oil to flow through unfiltered.

Unfiltered oil will readily lead to excessive wear in the engine since wear-causing particles pass through the filter uncaptured and are distributed throughout the engine with the engine oil.  The reason why it’s important to change the full-flow filter in this way, halfway between oil changes, is that there’s no way of knowing whether the full-flow filter has become clogged after, say, five months.  However, by changing it regularly every three months, even halfway between oil changes, you can be fully confident that it will never become clogged.

Bypass engine oil filtration is a far superior method of filtration which removes the overwhelming majority of contaminants from the engine oil so that the oil remains very clean all of the time and only has to be changed every 50,000 kilometres (30,000 miles).  For more information about bypass engine oil filtration, please see the next section titled, Bypass Engine Oil Filtration.

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