Apologies - didn't mean to be rude...just asking for more info or you'll get a response like Honda's that basically says get under there and look. For what it's worth...
What is ZDDP level of Castrol GTX 20W50?
"The previous owner of my car was pretty religious about oil changes and used castrol GTX only. Well - my exhaust valves were all worn and had to be replaced. The case was stained amber color, my layshaft bearings were showing copper, my cams were pitted. 5 rocker arms had to be replaced because they were showing too much wear. This is on an engine with 56K miles."
""Thank you for contacting Castrol,
Castrol North America does not manufacture GTX in a 20w-40 grade.
The latest API SM/ILSAC GF-4 category calls for reduced Zinc and Phosphorus levels to allow extended catalyst life in current model vehicles. There appear to be field issues associated with the SM/GF-4 oil's level of antiwear in the classic car engines known as flat tappet cam engines. The current late model passenger car engines are not flat tappet cam engines and have no reported field issues related to the level of antiwear chemistry in the SM/GF-4 oils.
Product Recommendations for Flat Tappet (Solid Lifter) Cam Engines:
Castrol Syntec 20W-50 (Recent reformulation identified by "Recommended for Classic Cars" text on back label) (min Zn = 0.12 = 1200 ppm)(full synthetic)
Castrol Consumer Relations"