Two-stroke motor oil, why is so different from the normal (4-stroke) motor oil?
2-stroke motor oils, like most other engine and gear lubricants, are composed of base oil plus an additive package. The additive package can be as simple (only one or two components) or as complex (multiple components), depending on the requirements of the oil.
The motor oils would not different with any of the normal motor oils. In base stocks compose from 50% to 95% of the total weight of the 2-stroke engine oil. And, they also can be synthetic and petroleum based.
BUT, in the additive part of motor oil, in addition to what usually found: dispersants, antiwear, detergent etc., they stress on low ash or ashless compound. As you might know the 2-stroke oils are burned away in the combustion process, ideally no residual must remain after each firing. The residue is undesired if remain as built-up on plug, stick on piston rings and will eventually clog exhaust port. They are added in in proportion with fuel during burning. these compounds are complex and heavily effect the price of 2-stroke oils.
The two types of detergents/dispersants most commonly used in 2-stoke oil formations are Ashless and Low Ash. Medium Ash and High Ash detergents are not used in 2-stoke oils. Ashless detergents are normally used in low temperature applications and work well in engines where an excess of cooling capacity is available and power valves are not used. The formulation was later changed to a low ash type detergent because of the higher temperatures generated by engines.
These oils are designed for air-cooled high performance engines that operate under severe load/temperature conditions. Low Ash detergents can keep the deposits to a minimum at ring land temperatures as high as 400 degF.
When 2-stroke oil is kept in its temperature limits, it provides an adequate protective film between all moving parts. When that maximum temperature is exceeded, the oil film breaks down and usually seizure occurs unless another line of defense is added to the oil mixture. These are the Antiwear agents.