2 Stroke vs 4 Stroke Engines

Wrong variable. What matters is 2-stroke vs 4-stroke engine.

In the smaller engines, there is some economy in not having to design and build in an oil containment system and a method of metering the correct amount of oil. It becomes easier to just use roller bearings, and just fuel-oil mist to keep it from burning up. But as those crankshafts and bearings become larger, and heavier, the bearing cost starts to out weigh the oil metering costs. An oil reservoir and metering device for a 5 hp motor is about the same cost as a system for a 35 hp engine. But the bearing costs would be much higher.

With the small engines we normally use I agree. But if we see many big engines are two strokes and they use only fuel. The true is only the small engines like Blower, motorcycles, small pumps, small marine engines, and others use oil in the fuel. Bourke Engine, diesel engines and several others, include the biggest 2strok engines in the world use only fuel.

There are a number of 2-stroke designs where the charge is not compressed in the crankcase, or where the bearings are sealed from the charge. In these cases traditional bearing lubrication (eg. wet-sump) can be used, and oil does not have to be added to the fuel. For example, 2-stroke diesels always (as far as I'm aware) use some form of forced induction (eg. Roots blower or the like), so compression in the crankcase is not required, wet-sump lubrication is usually used and oil is not added to the fuel.

I recall seeing an interesting 'stepped-piston' 2-stroke, 2-cylinder motorcycle engine in the 1970s where the charge was compressed in a lower section of the cylinder instead of in the crankcase. It seemed like a good idea to me, but I don't think it ever made it to market.

Wikipedia gives a good overview on 2-strokes.