I called Castrol about this and there's been a reduction in the "ZDDP" component in the 20w50 GTX. According to the tech I spoke to, the only one they offer with the amount of "ZDDP" called for is Edge-Syntec Synthetic 5W50. I called Valvoline and they say their 20w50 VR-1 Racing oil(conventional)has all we would need. There's a lot of discussion about whether it's even an issue in an engine already broken in. Other brands that claim to be o.k. : Mobil 1 15w50 synthetic, Amsoil 20w50 Racing(check variety for details), Royal Purple synthetic 20w50 HPS#31250,Brad Penn 20w50 Racing (conventional), and Moss #220-815 20w50. Prices are from about $6. to $10. per qt. There are also additves available. Good luck, Bob