Have no idea what a "MTS5500 tractor " is, but since it's an engine problem, engine details would be useful.

Could be a leaking carb needle/seat, leaking gasoline into the crankcase and thinning the oil or a blown head gasket or??

Please post the technical model number of the tractor (found under the seat) or at least the engine Model number and Type number found on the engine itself.
Having that information will permit us to give brand/model specific help tips.
A couple of things come to mind right off.
It might be a blown head gasket or it could also be due to a leaking carburetor (leaking in storage) that is filling the crankcase with fuel.
More detailed help can be given after we know some numbers.