Filter Bypass and Anti-Drain Back Valves
Filters also have relief or bypass valves. These valves are set to trigger if the filter element is making too large a pressure drop. Normally, this would be because it is clogged, however on an engine with a very high flow oil pump this can also happen if the oil is very cold. Motorcycles do not have high- flow oil pumps. These relief valves are set for different pressures, and sometimes a tech or mechanic will tell you that it's important that the relief valve have the correct rating. It's not. These valves are very low-precision devices, and their pop- off values are different from each other even in identical filters of the same brand. When the relief valve is open, the oil is going around the filter element and not being filtered at all. If your relief valve ever opens up you're either using a really cheap oil on a really cold day, or you haven't changed your oil filter since the last time you saw Robert E. Lee. Because filters can clog up and saturate with junk, your oil filter should be changed about every 5,000 miles.
Finally, some oil filters come with an anti- drain back valve. This is not necessary in a filter which is mounted vertically, with the opening pointed up. However, most motorcycles mount their filters horizontally, so this is very important to us. This is typically a piece of nitrile or silicon rubber which blocks off the filter oil inlets unless there is positive pressure into the filter. After you have used a filter for a while, if it's doing it's job, the filter is full of these 20 to 50 micron particles which mean death for your bearings. When you turn off your engine, if the filter has no anti-drain back valve, whatever oil is in the filter will drain back into the oil pan, bringing with it a whole bunch of really evil junk. So, we want filters with good anti- drain back valves: the stuff that's in the filter should stay in the filter. To the right is a picture of the Pure-One and the Mobil-1 anti-drainback valves. Both filters use the superior silicon rubber. The Pure-One has a larger core and a convex shaped mating surface to guarantee an excellent fit. The Mobil-1 has a flat seal and a flat mating surface which do not inspire the same confidence. The Mobil-1 anti-drainback valve is in turn clearly superior to that of most other filters.
The people linked below who dissected many oil filters found that some brands have a really cheap piece of plastic that doesn't seal very well. The filters I have listed as top rated all have good anti-drain back valves.
I received an email from Suzuki of Victoria, informing me that a Hayabusa they took in for service showed no oil pressure. They found the cause was an aged anti-drainback valve on a Hi-Flo filter that would not open, thus cutting off all oil pressure to the entire engine. K&N filters are made by Hi-Flo, so these filters are also suspect.