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We are on the same page..

Note that in the USA, Castrol GTX is 800ppm zinc "Liquid Death" to motors.
(named by a wise man who lost $$$$$ to that little un-announced "formulation change" by Castrol)
Same issue got me, twice, before the truth became known.

Castrol GTX was once the go-to choice for ACVWs, even if the 20W-50 was the viscosity of~water after 3K miles.
No more Castrol for me, even if they are owned by Shell now.
They still owe me 2 engines.

If you are in an area where you can find it, you can get Wolf's Head 15-40 with plenty of Zinc and Phosphorous for half the price of Brad Penn. Here is a used oil analysis in my Citroen 2CV, I run it my VW as well.
http://benboyle.com/uoa.pdf

Very Interesting, while looking for a local distributor, a little Googling reveals that Amilie (the owner/bottler of the brand Wolfs Head oil) may very well make the entire Rotella line for Shell under contract.