Being belt driven flex as you describe is not a problem that would cause the oil to drain from the crank case.
Broken gaskets , failed oil seals , loose dip stick tubes , blown head gaskets , holes in crank cases , worn out piston rings will all cause the oil level to drop .
One of the big downsides of ash less synthetic oil is most do not burn & those that do not burn do not smoke as it blows out the muffler.
So where does the oil accumulate on your mower ?
A bad gasket will cause it to pool on the engine bay floor .
A bad bottom seal will run down the shaft, pool on the pulley then get flung over the side rails
A bad top seal will blow all over the sides of the engine because of the effect of the cooling fan.
And as Walt has already asked, we need the engine numbers, mower make , model & serial number .
We also need to know what you are doing with your mower.
consuming the oil in the sump is a very rare occurence, to have it happen twice makes we wonder if it is operator related.
Very hilly properties must be mown with a horozontal shaft shaft or two stroke engine
