Unlike four-cycle engines which have a closed crankcase, these lightweight engines use the crankcase as part of the induction tract, and therefore, oil must be mixed with gasoline to be distributed throughout the engine for lubrication. The resultant mix is referred to as petroil. The two-stroke oil is ultimately burned along with the fuel as a total-loss oiling system. This results in increased exhaust emissions, sometimes with blue smoke and/or a distinctive odor.
The oil-base stock is either petroleum, vegetable, semi-synthetic or synthetic oil and is mixed (or metered by injection) with petrol/gasoline at a fuel-to-oil ratio ranging from 16:1 to as low as 100:1. To avoid the high emissions and oily deposits on spark plugs, modern two-strokes, especially for small engines such as garden equipment, may now demand a synthetic oil and can suffer from oiling problems otherwise.
Engine original equipment manufacturers (OEMs) introduced pre-injection systems (sometimes known as "auto-lube") to engines to operate from a 32:1 to 100:1 ratio. Oils must meet or exceed the following typical specifications: TC-W3TM, NMMA, [API] TC, JASO FC, ISO-L-EGO.
Comparing regular lubricating oil with two-stroke oil, the relevant difference is that two-stroke oil must have a much lower ash content. This is required to minimize deposits that tend to form if ash is present in the oil which is burned in the engine's combustion chamber. Since the 1980s different types of 2 stroke oil have been developed for specialized uses such as outboard motor 2 strokes, premix 2 stroke oil, as well as the more standard auto lube (motorcycle) two stroke oil.
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