And so it should be. The term "synthetic" for a Group III base oil is deceiving enough, but to call it "full synthetic" is even more so. The deception of marketing. Should not be allowed. Call it whatever it is like "Conventional Oil +".or "Conventional HP".The thing with 0W-40 is that it has same cold viscosity as 0W-20, it is only hot viscosity that differs Also, in certain EU countries you are not allowed to market cracked mineral oil as "fully synthetic" unless it is actually made from PAO stock. If you buy Mobil 1 in US it is cracked mineral that is deemed "synhtetic enough".
0W-20 will be thinner when hot and thus provide slightly better MPG, but unless engine has very fine tolerances some of that runny oil might escape the piston rings/seals.