Pecon farm here in the Las Cruces NM area used to do that for frost control with WWII Lockheed PV-2 bomber.
Hal

James,
You can get the FAA record from the Aircraft Registry in Oklahoma City for $5.00. Google it to get the address.
Hal

Continental service bulletin CSB09-11A, that applies to the following Navion engines, IO-520-BB, IO-550-B, IO-550-G,
IO-550-N, IO-550-R recommends against cruise below 2300 RPM.
Hal

Bill, 5 year change interval is based on 59 years of owning many Navions (Bought and sold aircraft at our FBO, specialized in Navions). Erich, Probably a better way, but disconnecting lines at low point and letting drain is one way. Brake systems seem to be the only hydraulics where the oil gets rea...

Sorry James. No knowledge of that one.
Hal

Current hydraulic fluid is 5606. Changing filter and fluid once every 5 years is often enough.
Hal

The larger holes for the rear exhaust are part of the basket beef up for the higher horsepower continental engines (240 hp and up).