Recently had the blinking oil pressure light and shut down on my 520. Started searching on RS about switch availability to get one on hand prior to removing the tins. As it's listed as NLA from Toro, checked with OnanParts.com which has original type - two terminal (NO & NC) switch available (Onan PN 309-0558) for 81.00 Plush shipping.
Figured I'd verify the switch was bad before purchasing a replacement.
I applied low press air and both terminals checked good on my switch. Found the Oil Press. sense (Dk Blue) wire almost pinched in half by the tins which must be the cause. Spliced the wire and hope that's it.
The sender body on the switch has Stewart Warner, Hobbs Division and what looked to be a PN. Mailed SW Cust. Support with the info on the sw. body to see if a superceded PN or replacement is currently available. They do not offer a direct replacement 2 terminal switch set at 5 (+ or - 2 psi) as used in the 520. They do have a 3 terminal NO NC switch available end users can adjust by approx. 1 psi. PN is SW76579. Did a quick search and found it on line for 59.00 plus shipping from a parts vendor.
Have to ponder the possibilities three terminals offer...