Something to ponder on:

1) FULLY SYNTHETIC by definition is 100% synthetic base. This we know cock sure edy.

2) MINERAL by definition is 100% mineral base. This too we know cock sure edy and they're cheap.

3) SEMI SYNTHETIC is what exactly?

10% mineral base + 90% synthetic base = semi synthetic
90% mineral base + 10% synthetic base = also semi synthetic

For all the variations in price for the different semi synthetic offerings, it would be simpler to just buy the cheapest fully synthetic oil you can find coz at least you know cock sure you're getting 100% synthetic oil.