Which weight of Castrol syn have you been using? I used to be a died-in-the-wool Castrol fan, but when I learned about the court case they brought to legally call highly refined petroleum oil 'synthetic', I switched to Mobil 1. Amsoil would be my next choice.
I think Purolator filters are the best value in a high-quality filter. (The Pure One line was rated #2 overall (in both cold flow & filtration efficiency) by AC Delco when they had their super-premium UltraGuard Gold line.) But my own Used Oil Analysis results showed that I got no worse filtration from their standard filter, so I switched to that instead.
I run Mobil 1 0W-40 in the '02 Focus wagon's Zetec. It's fortified additive package (over M1 0W-20, 5W-20 & 0W-30) give it extended longevity in any engine. Based on several years' worth of Used Oil Analysis in my other cars, I'm completely comfortable running this oil in this engine for 2 yrs/20k miles.
"Help, Mr. Wiza-a-ard!!"