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I haven't used that formula of Castrol, but used Castrol in a former truck I owned for years without any issues.
Just about any of the top name oils are going to be good.
But the real issue is why won't your mechanic use Mobil 1 if that is what you request?
Any shop can get any oil you request.
Perhaps the Castrol is what he keeps in stock and will give you the best price on.
But he should also be able to use whatever oil you request.