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Let me add a little to what West and Dave correctly pointed out.

Synthetic is great in air cooled motors because of it's superior high temperature stability. Ditto arctic conditions and turbochargers.

However, the extended drain interval is contingent on full flow filtration. Only selected Senior cars have this feature.

As for leaks, Chrysler was having a problem with differential noise and authorized a drain and refill with synthetic. The side effect was leaking seals. And this was in a modern car. Don't ask me why.

Back when it first came out I did know a Mobil Oil executive who put Mobil 1 in his Packards. Old timers were shaking their heads.

For these Packards, given they don't have emission controls makes for rapid exhaustion of detergency so I would stick to regular oil changes of 2000 miles (as per Packard) or at least once per year.

Also, I found a TSB where Packard said to change the filter twice as often (4000 miles) when using detergent oil. I guess they had more faith in that filter than I do.

Let me add one more judgement call here. The question comes up "Why should I change the oil in a car that I use only a few times a year?". Very good question. As detergent oil absorbs moisture it usually turns reddish. So to check the oil simply drain it into a clean pan and look at the color.