You certainly won't do any harm with a synthetic oil, but if the older Datto motors have wider tolerances, you might start consuming the stuff as it blows by the rings and so on.
Operating temperature is also an issue: a 40-weight might be good for an Australian summer, but thinner oil would perhaps be better in very cold conditions.
Also, there's not much point in using a thin, synthetic oil on a motor with a lot of wear: you need a film of oil to damp down movement.