Brother,
Actually its v. long discussion, but I've a short answer as follows
Thicker oils like 15-40, 20-50, 20-60, etc. are NOT Synthetic oils. But different companies use different words to attract customers, like they will write on 20/50 oil as, "SYNTHETIC FORMULA", or "WITH FULLY SYNTHETIC TECHNOLOGY", or "SYNTHETIC MANUFACTURING FORMULA", ETC. ETC.
But infact we all know that 20/50 is not a Synthetic oil.
For your ref,
usually Fully Synthetic oils are v. thin. Like 0-20, 5-20, 0-30, etc.
Semi synthetic oils or partially synthetic oils are like 5/40, 10/30, etc.
There is no hard & fast rule as described above to show or to write. But this is generally what is happening here. Even Performa making Fully Synthetic oil but with 0/40 viscosity.
THE OIL COMPANIES ALWAYS HIDE TO MENTION THAT EITHER ITS 5% SYNTHETIC, 10% SYNTHETIC, OR 50% SYNTHETIC, and hence making money like hot cakes.
Cheers & LOVE.