Q. When a cold engine is first started, does oil flow through the filter?
A. It depends, but usually not. Cold oil is thick oil that pumps slowly and circulates slowly. To speed lubrication to the engine and upper valvetrain, cold oil will usually force open the oil filter bypass valve and flow directly to the bearings and other internal engine components.

The oil pressure bypass valve typically opens when the pressure differential across the filter exceeds 10 to 12 PSI.
Low viscosity multi-weight oil flows better at low temperature, so a 5W-20 or 5W-30 oil and synthetic oils will start to circulate much more quickly than conventional 10W-30 or 10W-40 oil. A 0W-20 oil is even thinner and flows with less resistance at low temperatures. But if it is cold enough (below freezing, for example, or below zero), it may take a few minutes before the oil warms up enough for the filter bypass valve to close. Once the bypass valve closes, oil flows normally through the filter before going to the rest of the engine.

If the oil filter isn’t changed often enough, internal resistance can build inside the filter causing a restriction. If the pressure differential exceeds the opening pressure of the valve, it will open and allow oil to bypass the filter. This is a safety feature so a plugged oil filter won’t starve the engine for lubrication and cause the engine to fail. But a plugged filter offers no protection whatsoever, and sooner or later contaminants in the oil will cause wear and damage inside the engine.

Q. Does the oil filter protect the oil pump from contaminants?
A. Not directly, but it does indirectly. The oil pump is the only internal engine component that is not lubricated with filtered oil. The pump sucks up unfiltered oil from the bottom of the crankcase and then sends the oil to the filter and the rest of the engine.

Consequently, dirt or other contaminants in the crankcase can cause wear and damage to the oil pump. Once the pump becomes worn, it can’t pump with the same degree of efficiency, causing a drop in oil volume and pressure.
Once the oil passes through the filter however, most of the solid contaminants (dirt, carbon, bits of metal debris from normal wear) are trapped by the filter media. “This is because the oil filter continuously cleans the oil as the engine runs,” according to Wix’s Brad Drake. “It is helpful to think of the lubrication system as a circuit that remains closed or uninterrupted while the engine is running.”

Dirty oil may pass through the oil filter once, but if the filter is doing its job, it should trap most of the particles that cause wear, protecting the pump from further wear or damage.

To maintain optimum engine protection, the oil filter should be replaced at every oil change, and the oil should be changed at the recommended service intervals or when the oil reminder light comes on.

The standard oil and filter change interval recommended by most vehicle manufacturers today is 7,500 miles for “normal” driving, which does NOT include short-trip stop-and-go driving (especially during cold weather), towing a trailer, prolonged idling, sustained high-speed driving during unusually hot weather, or driving on gravel roads. Because of this, many motorists are actually “severe service” drivers who should have their oil and filter changed more often.