Just as a point of information here:
Scottoil is water soluble but engine oil and chainsaw oil are not. It wears off the tyres faster than engine or chainsaw oil if it gets onto them. The thin oil allows it to penentrate the chain more easily. I would be sure the Scottoil company would have researched this well and the brand they created was in line with safety of the motorcyclist.
Used engine oil is the wordst possible thing to use. The characteristic of engine oil change dramatically when used in a combustion engine and it as the more it is used the more acidic it becomes.
I would not think acid based oil would be a good lubricant for a motorcycle chain