They were originally rated 24:1 for maximum engine life, back when oil was dinosaurs and not very good. We have had a significant improvement in oil quality and effectiveness, so in the later years tecumseh put out a service/owners note saying 40:1 is acceptable to reduce exhaust and plug fouling since they realized these motors mostly used on ice/earth augers and chainsaws are not going to be ran for thousands of hours.
So yes 40:1 synthetic premix is what you should be running.