Hi Al
If the oil in the oil tank is above the level of the bottom of the cylinder, then any oil that syphons back into the engine while it is not running can fill the crankcase and then seep past the piston rings and settle in the cylinder between the piston and cylinder head.
If you try to start the engine with oil in this space you could cause a lot of damage as oil , unlike air, does not compress.
It is important that if you have a high mounted oil tank, you make sure the engine will turn over before trying to start it, and if there is a lot of oil in the crankcase, then this is pumped, by blow-by pressure, back to the oil tank.
On a completely separate issue, if you turn the prop backwards, there is a chance, because you are turning the oil pump backwards as well, that you could draw air from the crankcase into the oil system, and because air is compressible and oil isn't, the hydraulic tappets can collapse and not work properly.
I hope this helps explain it.
Cheers Mark
