oil pressure is a strange thing. Most people have the understanding that motors need a certain level of oil pressure to keep running, and to a point, that is correct, but, The original motors from years ago ran without a pressurized oiling system. In the old days, motor lubrication was done mostly by splash. There was oil in the pan, and the crankshaft would make contact with it (or swing close enough to force some of the oil in the pan out of the pan), and kick the oil up into the motor to lubricate the high wear contact points. That system was used pretty effectively up through the 50s in some motors, but as the power levels increased, it was discovered that pumping the oil through the motor was more efficient and the pressurized system became the normal. These days, many of the small one cylinder motors (like your lawn mower or weed eater) still use the splash system of motor lubrication, but nearly every multiply cylinder motor uses the pressurized system. Generally speaking, a pressurized system requires a minimum amount of pressure to keep the motor operating because most high wear contact areas are no longer open enough to get much oil splash, and most crankshafts no longer make contact with the oil in the pan (it takes away HP). When the pressurized motor does not maintain enough oil pressure, the high wear areas don't receive enough oil to properly lubricate the critical parts and the contact surfaces can wear extremely fast. as one might imagine, those wore out high contact parts begin to fail and those failing parts stop the motor from running, either by a slow grinding halt, or a fast explosion. The time frame for that to happen largely depends on the original clearances the motor was built to. The more modern motors are built to a tighter tolerance and the failure often happens sooner then the older looser tolerance motors. Also, the minimum oil pressure needed varies between the manufacturers, and even between specific motors within the manufacturers.

Standard operating procedure for me was to always run a mechanical oil pressure gauge on any motor I had any question concerning oil pressure. The old Mopar motors will run a long time on pretty low oil pressure, I've seen them run for years with less then 5psi at an idle when at normal operating temp but that low oil pressure usually jumps up as the rpm increases. Chrysler considered 10 psi per 1,000 rpm to be normal and adequate for performance motors.

The down side of all this is that some of the electric sensors won't function well with low oil pressure, many won't function well below 10 psi. If the motor doesn't have enough oil pressure to make the sending unit happy, it won't function. You could pull the power wire from any ignition powered wire (on or off with the ignition switch) for your choke rather then off the oil pressure switch, as long as the choke wire runs through the little procyclin terminal thing before it is connected to the choke spring. Gene