I think Grumpy was on the money with the smoothness argument.
The 1973 catalogue seems to say the most (that I can find).
Interestingly, they recommend BP Outboard Zoom (50:1).

- runs smoother
- runs cooler
- longer life
- needs less oil

https://www.outdoorking-forum.com.au/forum/ubbthreads.php?ubb=showflat&Number=62024#Post62027

All very interesting.
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JACK.