oil of that era was still DINOsaur oil. if one wants to try a synthetic there is the option of a synthetic blend.
The language here (from Ghibli lit) seems to leave open the possibility that Sint was a blend. Image Unavailable, Please Login I poked around on-line, and could not find any evidence that Mobil Super was full synthetic; the first ads that I could find that implies full synthetic Mobil is 1974. Similarly, UniFlo ads from that era don't say they are full synthetic. None of that is definitive of course, but the 1974 Mobil ads make a point of saying that it is full synthetic, so I would assume that any oil that was like that would make a point of mentioning it...
