I've used Castrol GTX 10/30 up until I had some work done last winter. Since the minor leaks were fixed at that time, I've changed over to Castrol Syntec 10/30.
My theory is as much as you pay for synthetics, just pay less for regular & with more oil changes, always have FRESH oil . With that being said, I change mine every 600 miles.
Ok, well here's the way I offset the cost of synthetic oil. I get the oil on sale, which is usually reduced by 25% or more. I only drive the 7-Up occasionally, so I change the oil and filter every 6 months. It's still less miles than a recommended change interval, so it's all good. Besides, synthetic oil is a better lubricant than regular stuff.
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I do use regular GTX 10/30 in my 68 though.