The 30-year-old Thornhill race ace placed sixth and seventh places in the day’s two 25-minute races.
He said: “Both races were a little frustrating as I was fighting an imbalance which badly affected the car’s handling. We don’t know the cause of the issue but the team will be working hard on rectifying it for the final races at Brands Hatch.
“The Porsche was a real handful, especially through right handers, I could barely turn the steering wheel without the rear snapping away from me.
“Nevertheless, I really enjoy Donington Park. I had my first ever test there, in a Renault in 2008, so lots of special memories.”
Wylie Ross signs off from his busy multi-championship season by contesting the final two Porsche races at Brands Hatch on October 24.