If you are talking about the Dynamo mini bikes...50cc 2-stroke.
then it does not take oil in the crankcase, but rather in the gearbox.
I have the original manual, it called for 30wt motor oil. But you do not want modern car oils because of the wet clutch.
I would use a good motorcycle engine oil or transmission oil.
The Motoul 10-30 Transmission would likely work quite well.
I do not believe it needs ‘extreme pressure’ like some gearboxes (vintage OSSA comes to mine)