Oil pump pressure regulator

Bill, I don't disagree with anything you just wrote, except to say that the ideal level isn't really a "practical lower limit", almost by definition of those terms.  Anyway, even after sitting for weeks I have to burp the engine to get the level in the tank from reading "low" back up to its normal level (¾ up the flat of the dipstick).  I have never had it go any higher – nothing much ever seems to flow down from the pump side. Losing prime on the pump has never been an issue.

I'm going flying today, as it's finally sunny and into the mid-60s in the Hudson valley (open cockpit here).  I'm bringing my tape measure, just to quantify exactly how low my tank is, for anyone who cares to know.  

I am still hoping that the Rotax folks will eventually weigh in on that 400mm number.

Your Engine may very well be close toleranced and the backflow rate might be as slow as the proverbial Molasses-in-January, but somewhere out there is an engine that is running on Condition, 1000 hrs beyond TBO and can backflow an ounce a day.

The engineers need to design and recommend for these worst-case situations.

If you are Open-Cockpit, I will assume that you are also a Tail-Dragger.

If the tank is behind the engine, the Tail Down angle will make the tank even lower.  Measure from the ground-up and subtract the difference.

Obviously your installation works for you without issues but Rotax needed to put a number on it and 400 was the number.

It will be interesting to see if 200 was supposed to be the number and it took 30 years for someone to notice the discrepancy.

Well, the answer on mine is a little less than 8" below the oil pump, which I guess is just about 400mm below the prop shaft.  Go figure.  :)

(No, it's not a "tail-dragger)

"If the oil tank is located higher [than the prop shaft], oil might trickle through bearing clearances into the crankcase during longer periods of engine stop. If fitted too low it might damage the oil circuit."

It seems a bit odd that the IM is very specific about why high is bad, but quite vague on why low is bad.  Damage the oil circuit how, exactly?