Hi KenB . . .
I think that castrol makes good products - and not to bash - but I don't think I like the marketing. Castrol Syntec isn't what a chemist would call 'fully sythetic' oil:
Motor Oil Viscosity in Layman's terms As I understand it Syntec is a Group III petroleum based oil with sulfur and other impurities refined out. It still has viscosity additives in it - which means it remains effective until the additives wear out. I wish they would just market it as what it is. Highly refined longer lasting oil that as result costs a bit more:
Old forum discussion on Group III oil. Frankly, I think it is hard to go wrong with modern oils in classic cars because we don't really push the enveloper in these 'leisure' driven cars. Most hobby cars get more frequent oil changes then daily drivers - and go way less miles. Begging the question why invest in long life oil?
As for multi-grades - if one is not cold weather starting then the second number is all that matters.
I run Traveller's straight 20 in my 216 all season. And store it for the winter with cheapest 10W-30 I can buy (for cold start ups through the winter) - as I usually run it up to operating temperature every two weeks. With salt on the roads during Ohio winters there are no short spins.
regards,
stock49