I've owned a couple of Rotec R2800's for several years. There are several causes for the oil "leak" issue.
First, gravity is the main culprit, of course. Second, fluid will seek its own level. If the oil tank is located above the centerline of the engine, oil will drain into the crankcase. From there it will spill over into the cylinder bases. Some engines have cylinder bases that extend into the crankcase forming a dam of sorts. However, the Rotec has notches in clyinders 4 and 5 to allow for clearance of the connecting rods. This allows the oil to run into the "jugs".
There being no such thing as a perfect seal, eventually oil will seep past the rings and into the combustion chamber. One of the lower cylinders will either have an intake or an exhaust valve open. Oil will drip out of the intake drains or the exhaust.
In addition, the engine has a scavenging issue. The sump tank needs to be larger capacity to allow for crankcase drainage after shutdown. It's normal to find 2 -3 quarts of oil in the tank even after a scavenging procedure. The addition of the microfilter in the line from the sump to the scavenge pump lowered the overall scavenging of the engine. It seems to be adequate, but a larger capacity would aid in all around lubrication system performance. Oil temp. would run lower, better scavenging at low rpms--less oil in the sump.
If you look closely at the location of the sump fittings, you'll see that the cylinder drain interconnects T into the sump at the very bottom of the tank. Now remembering that fluid seeks it's own level and that there is most likely 2 - 3 quarts of oil in the tank, where do you suppose the oil in the tank will try to to go? Yep, back to the cylinder. It would surprise you how much hydraulic pressure is created and how little pressure is requires to backflow. The tank should be a little longer which would let us lower the crossover drain input to the tank. Jacobs had the same problem on some of their engines. They addressed it by adding an additional in series tank.
Most radial engine manufactures addressed the problem of "seepage" by incorporating check valves (known as seepage valves) in the oil pump assembly. That removed the need for a tank shut off valve and the risks involved with it's use. A hi (stainless) quality inline check valve installed at the oil filter block inlet would accomplish this. A 10 psi unseating pressure should be adequate. The oil pressure in that line is very high and will pose no problem in oil circulation.
Sans such system, a shut off valve is necessary. I'd recommend installing the valve at the tank outlet with a Bowden cable to the cockpit. A microswitch in the starter solenoid line is another good idea.(I do not introduce the ignition during start until I see oil pressure. More than once it's saved my bacon.)