Also, Syntec Blend is probably thicker at higher temps than
Mobil 1. If you look at the manufacturer spec sheets, you'll see that even 5W30 GTX has a thicker viscosity at 100 dec. C than Syntec Blend or Syntec. Must be the parrafin wax that makes it thicker.
Why would you go from a synthetic to a blend anyway? Especially in the winter. If you want Castrol synthetic, use Syntec.