Jeff,
The OPSS is an oil pressure safety switch. The electrical supply to the fuel pump goes through the switch. If you lose oil pressure the fuel pump shuts off and the engine then runs out of fuel and also stops. This switch is required by US Coast Guard - not too sure for Canada. Regardless, it is a sensible thing to have as it protects your engine in the case of oil pressure loss.
https://www.amazon.ca/Carter-A68301-.../dp/B000CSWR10
you need to check which terminals are open with no pressure and close when there is pressure. Put the switch in-between the +ve terminal on the coil and the fuel pump.
Polishing filter - they are very cheap and easy to install so yes.