The oil filter return line on a 216 or a spray oiler 325 plumbs into the rocker arm oiling system, downstream of the oil distributor valve. It's normal to have a small amount of pressure on that port. Pressure-oiled 235's have the filter return going back to the crankcase. Since the main oil gallery pressure is always higher than the rocker arm oil line, oil will still flow through the filter, either at idle or cruise speed.
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!