Full synth is naturally multi grade so it doesnt need to have as much viscosity improver added, but it will still loose some of its multi grade ability.
Some 20 /50 dino oils drop to a 30/40 PDQ and are almost a straight 35 after 5000 miles, but a good synth rarely gets much below about 25/45.
Which is why some people recommend a 15/40 synth instead of a 20/50 dino - after about 1000 km they will both be the same thickness when hot anyway
So best to use an oil with as little viscosity improver's as possible, either a quality dino matched to the prevailing conditions, as detailed in the bikes handbook, or a good quality synth, which will probably have a better additive package too.
There is a dedicated oil forum, Bobistheoilguy, and it is unique in that some of the posters are industry pros who actually know something about oils in genera, the oils which are currently made and sold, and that knowledge is almost exclusive to that forum.
It has a motorcycle specific forum, plus an archive which contains more information than you will ever need on the subject.
