Can you use synthetic 10-30 in a 1968 vw beetle?

Sure you can. However.... if you're not running a bypass filter setup then the benefit of longer change intervals is not there. Whether synthetic or regular it's going to get dirty enough to change in the same time.

For all around protection the very best oils would be ones with the high zinc levels, whether dino or synthetic. Typically called "racing oils" because of the zinc additive (zinc protects flat tappet engines like the classic vw) damage catalytic converters. Some of the better known, high zinc oils are Amsoil, Brad Penn and Valvoline VR-1.

To sum it up: Run a good, high zinc oil, either dino or synthetic, to protect your classic engine. With a bypass filter you'll get a longer time between changes besides the other benefits of a filter. Hope the helps....