Time for the 5w50 just so you know it has extra protection, then you can chase it down, may be the turbo. But, it could be the ethanol as well.
^This. Definitely move to 5W50.
2x 5L jugs of Motul Sport 5w50 just arrived today. I only have 50-75 miles on the new Castrol, so I'll just dump it and take the $ loss. Fortunately, it's relatively cheap!