NOTE: This article is not intended to be a full, detailed, explanation of synthetic and petroleum based oils, NOR, is it intended to be and "End All" to any discussion regarding these oils.
It IS purely an informative based article based on experiences from RK Tek.
Many ask why RK Tek Inc. does not recommend synthetic oils in 2 stroke engines?
First, a little history on why synthetic oils were introduced.
Synthetic oils were invented to help combat the break-down that a petroleum based oil experiences under a high heat application/environment (ie. Hot running automobile engines) In other words, under extreme heat, the petroleum based oil can break-down and cease to properly lubricate the engine's internal components. This, obviously, can result in catastrophic engine failure.
Synthetic oils can be chemically engineered to avoid break-down under extreme conditions. They can also be engineered to chemically bond to specific metals that are present inside certain engines. They also stay cleaner.
More detailed info on synthetic oil can be found at http://en.wikipedia.org/wiki/Synthetic_oil.
OK, So, it sounds like this synthetic oil is the clear winner for high heat applications.
So, why does RK Tek not recommend it in the 2-Stroke Engine?
The flow and environment of oil in a 4-stroke engine is much different than a 2 stroke engine.
Let's break it down a little......
1) Oil is the main source of cooling. Its primary purpose is to flow onto, into, and around the piston, cylinder, crank components, cam components etc. etc. etc.. It keeps these internal components cool and lubricated and helps avoid excessive heat and friction.
2) Oil is continually re-circulated throughout the engine. The oil is not given much of a chance to cool down until the engine is turned off. The oil is always being heated whenever the engine is running.
3) Oil is constantly be subjected to "dirty" entities and will capture these entities and become more impure.
There are many more.. but we will stop here for now..
1) Oil is NOT the main medium for cooling the engine. The main cooling agent is the fuel.
2) Oil is held in suspension with the fuel. Meaning the fuel is the vehicle for the oil's travels.
3) Oil is NOT re-circulated. It is introduced into the engine, does its job and is ejected out of the engine. This process repeats over and over. This is why you have to add oil after every few rides. Simply put, the oil does not stick around, it is consumed.
4) Oil is NOT constantly subjected to the internal engine components. It is not super-heated.
OK, this touches on a few key differences between the 2 types of engines.
Let's now examine the functions of the oil.
1) Oil remains in the engine and is pumped to the engine's components. It is a bath system that relies on dedicated pathways and "Splash" techniques to reach the required components.
2) Oil is constantly heated whenever the engine is running
3) Oil is re-circulated and requires frequent purging (oil change) in order to keep it clean.
1) Oil falls out of suspension in order to perform most of its duties as a lubricating agent.
2) Oil is NOT re-circulated. It is a waste product. It gets in and gets out.
3) Oil temps are kept very low due to the constant presence of fuel.
4) Oil forms a light film on the cylinder wall to keep the piston and rings from seizing to the the bore. In fact, and this is a strong statement.. If the oil film never breaks-down/fails you can not have an engine seizure. you can still have engine failure, but not a seizure.
5) Oil lubricates the connecting rod, wrist pin, wrist pin bearing, and crank bearings. These bearings are also kept cool via FUEL constantly passing over them. Of course, the outer crank support bearings are not subjected to the fuel.
So, what does all this mean???
It means that the oil of a 2 stroke and 4 stroke engine are introduced to the engine very differently and they are subjected to much different environments.
It would stand to reason that these oils are engineered very differently and they are.
Synthetic Oils are engineered to withstand high heat without chemically breaking-down like their petroleum based counter-parts.
2 Stroke Oils are NOT subjected to the same level of heat of that of a 4 stroke oil.
Fuel, not oil, is the main cooling agent in a 2 stroke engine.
Oil, not fuel, is the main cooling agent of a 4 stroke engine.
Due to the method in which the oil is introduced, and the fact that it is not re-circulated, a synthetic based oil is not required to avoid oil break-down. The elements that mandate a non petroleum based oil are simply not present in a 2 stroke engine.
Synthetic oils are much harder to "burn" Usually, they do not burn. This is a problem is a 2 stroke engine.
Some Synthetic oil molecules CAN BE larger is size and can NOT penetrate (get into) the tiny areas that require lubrication (like lower rod bearings etc.)
Oil penetration is a necessity for bearing to survive under extreme load/heat.
There is one argument that many use. It is something like this: "Well, my exhaust valves stay much cleaner when using synthetic oil". Interesting... Ask yourself, What does clean exhaust valves have to do with bearing and cylinder lubrication?? Answer: NOTHING!
Please do not confuse clean exhaust valves with better lubrication. They are NOT related. In fact, the fact that the valves are clean lends credibility to the fact that the synthetic oil is NOT burning/combusting or penetrating small areas but rather just "gliding" over parts without impregnating in the parts (needed).
Oil must attach itself and penetrate into the "voids" of your engine. The smaller the oil molecule, the easier it is for this to happen...too large of molecule (synthetic) and it can not enter the needed/required areas. OIL is your engine's life blood!!
1) BRP/ Ski Doo quit recommending synthetic oil to be used in their snowmobile engines in 2006.
2) Synthetic Oil can NOT be used to properly seat piston rings. This is a KNOWN fact to the manufacturers of synthetic oil. MOST have a non-synthetic "Break-In" oil that is used to properly break-in a new engine.
3) AND the #1 reason why a synthetic oil is not as good in a 2 stroke engine--> IT DOES NOT FULLY COMBUST!
Your engine takes the energy from the combustion process to perform work on the crank train. This energy conversion is "fueled" from the Fuel/Air/Oil Mixture that it has available during the combustion process.
The Fuel and the Air will combust and be converted to energy.
The oil (if Synthetic) will not be converted to energy because it will usually not combust. So, this "oil" is taking up "space" in the head that will not aid with any energy transfer. Basically, DEAD WEIGHT!
Mineral oil WILL combust and aid in this conversion of energy! So, its energy is utilized!