Oil variety & marketting over the last 20 years has meant there is a lot of miss conception & miss representation about the oil grades on the market.
Without going into too much detail, here is the run down:
group I: mineral oil (think castrol GTX of the old days)
group II: same as above with less impurities (think current cheap oil)
group III: same as II but with more impurities removed (think high quality mineral oil)
Goup IV: synthetic based fully synthetic oil (think expensive oil: AMS oil, Mob 1)
Group V: as group IV but with a different base element

Because of cost and improvement in oil purification processes, gp III are a lot closer to IV & V (fully synthetic) but cost noticeably less.
Now Castrol was cheeky and started marketing an mineral based oil as synthetic which didn't go down well with Mob. After some legal wrestling, the court decided that the Castrol could brand their oil as synthetic cause it was synthetically derived from a (gp III) mineral oil.
Currently with 2~4 exceptions, all so called synthetic oil use gp III mineral base oil which is then synthetically grown/ manufactured to provide the desired properties.
The biggest con which the motorcycle world has fallen for is the semi synthetic.
By definition a semi is a mix of mineral oil with up to 35% synthetic oil. Most producers put in between 15~25%. In addition, they generally use gp II mineral oil which is very cheap. So the outcome is very cheap product to produce which they can sell as premium grade with premium prices.

Best advise is that if you really want to use semi (for whatever reason) get good gp III & IV/V oil of the same grade and mix them to a 15~35% level.

Sorry this was so long but believe me it is very condensed (relatively speaking)