Hi John H,
I can imagine water accumulating in the low point of the system, but I would think proper oil pressure (actually suction, at the cooler) would keep all liquids moving pretty well through the cooler. Water is not that much more dense than the oil, which obviously needs to flow through there pretty freely.
In my case, the low point of my oil system is my oil tank. I have never seen any water in it, as I mentioned earlier in this thread. But the tank is exposed to the atmosphere, whereas an oil cooler cannot be.
My schedule for actually disassembling/cleaning the oil tank is every 200 hours. That's mostly to check for lead sludge if you use much 100LL. I only use 100LL when flying away from home (and then I add Decalin), so my oil tank has been quite clean each time I have checked it.
CLEANING THE OIL TANK
General note
This procedure is optional and requires purging of the oil system. See Chapter 12–20–00 section Purging the oil system. If using leaded fuel it is required to clean the tank every 200 flight hours. It is only necessary to clean the oil tank and the inner parts if there is heavy oil contamination.
(MML: 12-20-00, pg 49)
In my system, however, with the oil tank below the engine, it cannot be any more necessary to do a purge when removing the tank than when simply draining the oil during an oil change. In both situations both the IN and OUT oil lines are left dangling in air (perhaps if the tank is higher up such a purge might be necessary).
Where you do want to be quick to avoid having to purge, I think, is when changing the filter. Have the new one ready to go on as soon as the old one comes off. Same goes for the magnetic plug check.
I agree with most of what Abid just said; I think he's right that if you run it up a little once you have landed (not much air streaming past your cooler) it should get your oil temp up to a good enough level for dealing with water in the system. Remember that the minimum temperature for simply running the engine is only 50C, and that's mainly for proper pressure/viscosity.
T