Personally, I use Penrite 2 stroke oils. 1.5 years ago, there was the semi-synthetic 1l bottles out for $1 more than the 'greenskeeper' full mineral oil. Did notice less smoke, but don't know if all the things about preventing carbon build up under the rings is true. Semi-synth or not, they all seem to have about the same sulphated ash content, but the full synthetics have a lower flash point, meaning less smoke.
I have almost finished that bottle and recently bought the Penrite MC-2st full ester synthetic oil - $22 for 1 L - figured I might as well give it a go. If it can be used for highly stressed bikes, should be great for garden low compression 2 strokes. Same probably can't be said for the super high octane fuels they use.
However, 2 anecdotes:
A mate bought a GMC trimmer in 2003, runs the castrol self mix at 25-1, never serviced it, cleaned air filter twice, never checked or changed the plug. The coil was buggered last year, so I replaced it with another used one, did a full service. Replaced the plug, but the original cleaned up fine ( bearing in mind 1200 sqm block with lots of lawn fortnightly use), little carbon in the muffler, and only a little blow by down the piston. Runs great.
A previous neighbour only has the small lawn at the front of the house (to the point where starting the Victa was more an inconvenience than mowing). He told me he only sometimes added oil to the mix - and it was usually the left over canola or vege oil the missus had used in the kitchen (put through a strainer.
And then you see the well looked after engines that just go bang for no apparent reason...
