On a four stroke engine, the crankcase is sealed from the combustion chamber by the rings in the piston, and the crankcase is open to the atmosphere (through a breather) so that no pressure is built up from the descending pistons. The engine has an oil pump and oil passages through out so that the oil is distributed to the crankshaft bearings, connecting rod bearings, cylinders, valve train, etc. The bottom ring on the piston is an oil scraper to that prevents the oil from migrating past the rings and into the combustion chamber to be burned.
On a conventional two stroke engine, no matter if 3.5cc or 800cc, The cylinder is open to the crankcase via the inlet ports and the motion of piston and hence changing volume of the crankcase is used to pump the air/fuel into the crankcase and through the inlet ports into the combustion chamber. Due to this configuration, the only way to effectively lubricate the crankshaft bearings, connecting rod bearings and cylinder is to either mix oil with the fuel, or inject oil into the air/fuel inlet tract or crankcase.
There have been very large two stroke diesels that use the same crankcase lubrication system and superchargers to pump the air/fuel mixture into the cylinder. Such a system would not be practical on a model engine and in terms of power to weight ratio, a conventional two stroke engine can't be beat.