Hi Neil,
As I understand it, the 250 motors went from 33:1 to 50:1 at the time the Supa5 was introduced. prior to that the main bearings were lubricated by gearbox oil so the mixture ratio was only concerned with lubricating the bigend, piston and small end. Don’t think the piston or big end were changed significantly but early motors had bronze bush small ends whereas the later motors had a needle roller bearing. It was this that supposedly allowed a weakening of the oil ratio. Other factors may have come into play, improvements in oil quality and possibly a marketing influence to make the bikes seem more modern in the Western world.
For what it is worth I have always run my ES250/2 on 50:1 even with a 4spd motor but this does have a needle roller small end. It has never seized or given me any problems related to fuel mixture – plenty of electrical problems though but I don’t think lack of oil was the cause!
The 125/150 motors went from 33:1 to 50:1 around the same time as the 250 motors as far as I recall but on what basis I don’t know.
However, in truth the difference is marginal in terms of cost and smoke reduction so I would tend to err on the side of caution over reducing the amount of oil in the earlier engines unless you know for sure the condition of your motor and what type of bearings it is using.
Peter