Yes Bill, I saw that it has an oil PRESSURE switch. But, as you stated, here in the US of A we don't usually see shutdown systems tied to the oil pressure. Having a shutdown system that kills the engine upon a drop in oil pressure requires that a provision be made that will permit the engine to make a spark in the ABSENCE of oil pressure in order to allow the engine to be cranked up from a dead state. Provisions for this can be a manual over rule switch that the operator must enable during cranking, or more sophisticated systems can feature an automatic electronic timer that will permit the spark to be present for about a 10 second period before arming the shutdown circuit. Once the shutdown system has been armed by either method, any loss of oil pressure will have the shutdown work and kill the engine. If.....this Ozzie configured machine has an oil pressure shutdown system, it MUST also have some kind of provision to allow spark during cranking when there is no oil pressure at first. Now, on the shutdown systems I am familiar with, there is a 10 second timer which allows spark in the absence of oil pressure to assure engine starting. After 10 seconds of time (beginning when the key is turned on) without an oil pressure signal to the shutdown module, the engine will be deprived of spark. On those systems, if you simply turn the key on and let it remain on for more than 10 seconds before cranking the engine.............you will never get the engine started because the timer has run down and enabled the shutdown module. In lieu of a bona fide wiring diagram by the machine OEM (not Briggs) anybody trying to diagnose this is fielding their best SWAG technique. I am just hoping to provide some food for thought on the subject in general, not knowing any specifics of circuitry on the machine to go by.

You want an automotive oil pressure switch that is intended to operate a "low oil pressure idiot lamp". This switch is Normally Closed configuration. You will also need to use a manually operated "spring returned", Normally Closed switch (push button type preferred) in the circuit (wired in series with the low oil switch) which you will hold down when you want to start the engine. Without the "push button" switch to manually overrule the low oil pressure switch for starting, you might have a longer crank time (to wait for low oil switch to open) until enough oil pressure created by the oil pump. The manual push button opens the circuit to defeat the low oil pressure kill function. When the engine starts you take your finger off the push button switch and your low oil shut down switch will kill the engine in the event of low oil pressure.

I'm not familar enough with your 4-stroke EFI Mercury Boat engine to advise for sure, so I'll hazard some guesses. How is your fuel system pressurized? Is it by electric fuel pump, or by a mechanical unit driven off a cam in the engine? If its cam driven, this is a possible source for a fuel leak directly into the crank case. Typical fuel pressure for injectors is about 50 psi. The fuel log remains under pressure after engine shut-down. If an injector does not seal well, it can weep fuel into the intake manifold until the pressure is relieved. Also, since the fuel tank is a closed system, a temperaure rise can keep the system under pressure and dump more than the expected couple of cupfuls or so that was in the fuel log and lines. If a cylinder was mis-firing, unburned fuel can work its way past the rings and slowly dilute the oil. If the crank case breather system was not working as it should, fluids and condensed gasses (getting past the rings) will build up in the crank case oil.

I'm having a new problem with my well water: when I'm running water the pressure will get to about 20psi, hold for about 5 seconds, then drop to 0! After about 2 seconds at 0 I can hear it kick on, and then the pressure jumps up to 20 and climbs back up to 50. I replaced my pressure switch and well pump in early August of this year (close to 4 months ago). I replaced the pressure switch myself with a Square-D 30-50psi with an Auto/Start/Off lever. The model is FSG2J21M4CP: https://www.amazon.com/Square-FSG2J21M4CP-Standard-Pressure-Switch/dp/B0002YSOPM That didn't fix the problem, so I hired a plumber to replace the well pump. He said that it looked like lightning had ran in on it. It also has a Wel Flo Pre-Pressurized Water System Tank, model # 24221808, that appears to be OK. What do you all think... faulty pressure switch?