Here is a brand new, never opened oil pressure gauge kit (includes gauge and install kit) to replace the vacuum gauge on a 520 with an oil pressure gauge OR add the oil pressure to any WH tractor with a Onan P216/18/20 motor. You can install the oil pressure gauge in place of the low oil motor kill sensor or in addition to that sensor (by using an easily obtained brass T-connector).
This is a mechanical gauge requiring no electrical wiring.
I got this as part of a WH gauge assortment and am selling off the extras.
This kit uses copper tubing instead of plastic tubing making it heavier duty (my view). I had a 520-H that used plastic tubing and never had a problem, but I prefer copper over plastic (if you prefer plastic tubing, it is easily obtained online or at virtually any hardware or big box store--and will work equally well with this gauge).
$25 includes shipping anywhere in continental US.