Hi Mike,
In my "bypass" filter setup, the oil pressure is not affected by the installation of the filter due to the tiny flow rate through the filter.
The oil flow is as follows:
Out the top of the oil pump
Into the inlet of the filter
Out of the filter and into a "fitting" with a small orifice.
Out of the fitting and into a hole I drilled in the block
The filtered oil flows very slowly through the filter, like a fast drip, not a stream. Remember, the pump only produces a tiny amount of pressure but a fairly decent flow rate. It also does not pressure lubricate anything. It provides a flow to the center bearing and to the roller bearings. However, splash lube can do just as good a job in this case.
Most listeroids have oil pumps that don't retain prime. That means there is no oil pressure at all when the pump loses it's prime. Rocketboys oil pump failed for many hours without any problems at all. The listeroid is really a splash lubricated design.
The filtered oil never directly lubricates anything. However, if you are very clever, you could use it to lube the #2 injection pump cam and roller. (a problem on the twins).
Amsoil sells a spin on type bypass filter. You could use just about any remote filter. I chose the Motorguard filter due to the 1/2 micron filtering capacity.
Rocketboy does not have a filter on his twin.