"Oil Milkshake" is normal associated with water in the oil, not fuel. Unfortunate condition for sure.
If you confirmed your not losing coolant (it doesn't take much to contaminate the oil), it's possible you could have water entering somewhere else. A prime other possible source is the upper cab cowl vent drain discharging onto the top of the valve cover instead of off to the side and behind the engine.
Does your early 235 have the two bolt valve cover with vents like this? Water from the cowl drain pipe, if cracked or installed and pointing in a straight down position, can drain onto the valve cover rear central flat area and trickle down into the vent, collecting inside your engine. It will mix with the oil when running and create the milkshake effect.
Let us know what you figure out.
