G'day folks,
The potential issues with modern 4-stroke oils in 2-strokes are mainly due to the differing additive packs between the oil types.
Modern lube oils consist of a 'base oil' blend, plus various additives. Something like the 1950s Castrol XL had bugger all additives, in comparison to the current Castrol XL!
Without getting into the chemistry, modern 4-stroke oils can be up to 50% additives, to boost properties such as: anti-wear, detergency/dispersancy, anti-foam, anti-rust, viscosity index, anti-oxidation; to name the major ones.
Not all of these 4-stroke additives function as intended in a 2-stroke engine with its 'total loss lubrication', and they create three main potential problems;
1. Additives
may not mix into, or remain dissolved in, petrol; they may also form gums
2. Some of them are metal-based, and will leave significant solid ash on burning, which can be abrasive and/or leave deposits in the combustion chamber - 2-stroke additives are ashless or low-ash.
3. Excessive smoke, as these oils are
not designed to burn cleanly, unlike 2-stroke formulations
That said, it can be quite possible to get away with using some types of 4-stroke oil in
low-output 2-strokes, such as mowers. I wouldn't use them in brushcutters or chainsaws, though.
And they're
not advisable in any really high-performance 2-smokes, e.g. karts, motorbikes, outboards/jetskis, any Rotax engine, etc. Recipe for disaster, right there.
If you
must do it, it'd actually be better to go for cheapo generic brand oils, as for sure they will have less additives in them, to save cost.
As far as 4-stroke mower oils go, the main reason for using SAE30 monograde in sidevalve engines [older Briggs etc] is just to minimise oil consumption. Multigrade oils are fine from the lubrication perspective, but in Oz, go for one with at least a SAE20 cold viscosity [i.e. 20Wxx].
As far as cost and low-end 2-stroke oils are concerned, there are two main reasons for their higher cost;
1. Economies of scale - car and truck engine oils are produced in vastly larger quantities - by far the dominant factor. And small retail packs are invariably the dearest option!
2. The additives that
are used in 2-stroke oils are more expensive than those in 4-stroke oils, though this affects top-end products to a much greater extent.
