There is a hole about 1/16" in diameter about halfway up the center standpipe or a little higher. That hole controls oil flow through the filter so there is not enough flow to drop the oil pressure. With 20-30 PSI of pressure, plenty of oil will flow through the filter, particularly with the engine at normal temperature.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!